Vanguard and Rangers
Before we begin, I just want to quickly address the reasoning behind combining these two articles into one; the units are incredibly similar with the only differences being their stock ranged weapon, their cost and one special rule being exclusive to either unit. To avoid confusion, any reference to both units will use the generic "Skitarii" label.
Capabilities - I feel the key to understanding where and when to apply a unit in any given situation is informed by knowing their full capabilities off by heart; here is a look at what this unit is capable of.
The Skitarii possess a fairly generic profile in keeping with their human - albeit augmented - origins. They are fairly durable in the context of what can only be billed as a light infantry unit, what with Toughness 3, a stocky 4+ armor save and Feel No Pain (6+) allowing them to ignore just over half of all wounds they suffer that don't ignore the protection provided by their War Plate. However, standard weapons will force easy saves on them due to their low Toughness while ignoring that saving throw isn't terribly difficult; don't expect them to withstand much punishment on a model-to-model basis. They are not too prone to failed Leadership-based tests given that they have a high base value of eight or nine provided their Alpha survives - and this can be further increased with a number of unique army wargear options such as Enhanced Data-Tethers. However, the lack of Fearless or even something like Stubborn makes them decent but not great objective holders or really that suitable to tying up enemies in an assault; keep this in mind if you want to use these cheap infantry squads for similar purposes to Synapse-backed Termagants or Plague Zombies.
As far as the Shooting phase is concerned, Vanguard especially are surprisingly potent given their slot and points cost; though their Radium Carbines and various optional special weapons have mostly low ranges, whatever does find itself in close proximity to them will quickly regret it. Their basic weapons can harm any model with a Toughness value in the game and force multitudes of wounds while Arc Rifles can rapidly disable vehicles in a single volley, making them a huge threat to any possible target short of Imperial Titans from the Age of Darkness. Though their effective damage output against non-vehicle units isn't necessarily amazing - ten Vanguard armed with Radium Carbines average around three slain Space Marine equivalents at 18" - the potential to roll lots of sixes to cause double wounds per individual wound roll can be enough to slay even mighty monstrous creatures with little difficulty. Having access to three special ranged weapons in a full squad of ten models or two at a minimum of five models is fairly incredible by human standards and lets Vanguard push the boundaries of what an Imperial Troops choice is capable of. The Rangers follow the example set by Vanguard with regards to special weapon availability and the raw power of those optional guns, but their basic weapons and thus role in a Shooting phase are rather dissimilar. Possessing Galvanic Rifles instead of Radium Carbines, Rangers can be seen as the Imperial alternative to Tau Fire Warriors with 30" range Rapid Fire guns that trade Strength for armor piercing and the Precision Shots special rule; Strength 4 AP4 makes them deadly anti-infantry units. Though they fire less shots than Vanguard and are incapable of harming any model with a Toughness value, Rangers have the distinct range advantage and as such are generally the better foot-sloggers if you want cheap objective holding units when faced with gun-line armies. They can trade blows at long ranges, but if you just want to hide the unit then the Vanguard are cheaper and pack a better punch if anything gets too close to them.
On the opposite spectrum of death-dealing, Skitarii are a situation-dependent melee squad that can punish other units of a similar pedigree or - in the case of Vanguard - combine with allies to really drive home an advantage in a hotly contested match-up. Their raw relevant stats consisting of Weapon Skill 3, Initiative 3 and a single attack per model are about as mediocre as it gets and give even Tactical Marines the edge, but there are a few outlying factors that somewhat make up for this. Before we get into those, I should address clearly that an Alpha bedecked with some type of pistol and an Arc Maul is an utter terror to all vehicular units due to the sheer amount of Haywire attacks he or she outputs, so keep that in mind if you think a Skitarii unit is incapable of doing anything truly worthwhile in close combat. Getting back on topic, the first of the supporting elements is that the Assault type on the Vanguards' standard armament and the supplementary Relentless special rule for both units allow them to fire whatever weapons they possess and still declare a charge, outputting the maximum possible attacks in both the Shooting and Assault phases of the same turn.
Next, all Vanguard in particular victimize their opponents with a potent Rad-Saturation aura that decreases the Toughness of any enemy unit locked in combat with the Vanguard by one point, meaning - for example - that the Vanguard are capable of harming natural Toughness 7 foes or wounding Space Marines and Necrons on fours rather than fives. This doesn't make them an independent assault force but it does nonetheless confer upon them a powerful advantage where other light infantry are concerned, but most important is the synergy between them and your proper dedicated assault squads - whether they be from the parent codex or others. The wording of the rule is such that you could even allow allied Tyranids to have an easier time of wounding your enemies; keep in mind that the Vanguard themselves can have a rough time actually making it into a melee in the same breath as an Imperial death-star (heh) so don't bank on this working too often. Lastly, as with all other units in the codex, the Skitarii are possessed of the Doctrina Imperatives that can severely alter their ranged and close combat effectiveness at your discretion; they can be used to form a powerful alpha strike in conjunction with friendly transports or to give them a defensive bonus in an assault. The Rangers by comparison have even less reason to appear in an assault, but the mixture of Doctrina Imperatives and Move Through Cover allow them to both close the distance better than Vanguard and still gain a Weapon Skill advantage over their opponents.
How to Equip Them - Outfitting a unit is often even more difficult than determining the number that you employ, so having appropriate context for each choice is key; this rounds out the most optimal ways to equip a squad.
Starting with the Alpha, there are a lot of different choices available for customizing your Skitarii ranging from melee weapons to unusual equipment. I personally wouldn't bother with a close combat upgrade for an Alpha because each selection is rather pricey and the unit itself lacks the reliable means to get into a melee, and truthfully does not want to be there save for Vanguard-specific combo-charging or tying up key ranged forces. The Rad-Saturation rule is nice in that case but consider that an Alpha pays the same amount of points for what is effectively a Strength 4 Power Sword as a model with better Weapon Skill and Initiative values in the form of a Space Marine Veteran Sergeant. No, the real value here lies in the Taser Goad and Arc Maul that capitalize on that -1 Toughness aura to inflict Instant Death on what would otherwise be Toughness 3 foes - keep the melee weapons to your Vanguard Alphas as Ranger Alphas lack the awesome Rad-Saturation special rule. Don't bother with the Radium Pistol unless you want the bonus attack when using a melee weapon as it is a straight down-grade from the Radium Carbine or Galvanic Rifle, while the Phosphor Blast Pistol and Arc Pistol are fairly priced for what they do - again, only use a pistol if you want to use the Arc Maul or Taser Goad.
What is really intriguing about Skitarii characters is the sheer depth of customization you can work with, including defensive wargear and even the vaunted Relics of Mars. Refractor Fields and Conversion Fields provide increasingly powerful invulnerable saves to an Alpha; I tend to prefer the former for its lower price and the fact that there is no potentially crippling special rule to its name unlike the latter. I'm not really a fan of Digital Weapons either as it maximizes the melee damage of a model that, as I've mentioned before, is still not ideally suited to that; the upgrade itself is rather expensive considering that it provides merely a single re-roll on a failed to-wound roll. In regards to the Relics of Mars, Arkhan's Divinator is an excellent value purchase whereas I would probably skip most of the others for an Alpha that can't make the most of their often melee-oriented abilities. Of note here is that the Pater Radium has amusing synergy with Vanguard and their -1 Toughness aura, while the Phosphoenix and Skull of Elder Nikola are best suited to units arriving via Drop Pod.
For the squad itself, Arc Rifles are generally the go-to special weapon because of the incredible anti-tank threat they provide to diversify the units' role as well as it being the cheapest of the three available choices. If cost and risk aren't determining factors for you - this is code for "are you using the War Convocation" - then the Plasma Caliver is the most powerful weapon by far as it murders not just light to medium vehicles, but monstrous creatures and darn near anything with Toughness 8 or lower. Though Skitarii are Relentless and can thus move and shoot the Transuranic Arquebus, a single AP3 shot that wounds on a 4+ and is Strength 4 with Armorbane will rarely ever do much damage at all even in decent numbers - speaking of which, these things are darned expensive and only have range as a true advantage over the slightly more expensive Plasma Calivers. Enhanced Data-Tethers are generally unnecessary in a sizable Skitarii force due to the presence of other Leadership-boosting items each vehicle utilizes, but the Omnispex is a truly powerful upgrade that can render Skitarii employing Plasma Calivers utter death machines on the battlefield. In the same vein as Enhanced Data-Tethers, be mindful of any Luminagen weapons in your army (they fulfill the same role as Omnispexes but affect more than one squad at a time) and plan the purchase of an Omnispex accordingly.
Best Uses - It is true that each unit in the game ultimately has one or a handful of tasks they are best suited for in a general sense; these are the most efficient uses of this unit based on their capabilities.
Though it is common knowledge by now, it still bears repeating that the absolute best use of Skitarii is travelling deep into an opponents' battle-line to deliver their high powered ranged attacks to specific vulnerable targets. Both Arc Rifles and Plasma Calivers allow the Skitarii to reliably take down vehicles, with the trade-off between the two being that the former can obliterate even mighty Land Raiders while the latter has little difficulty gunning down super durable Riptides. Though Interceptor is obviously a good counter to such a tactic given the fragility of Skitarii, the low cost of even a fully-sized and well equipped squad on top of their transport lends itself well to using them as your sacrificial advance elements to bait out Interceptor fire and devastate opposition that fail to appropriately neuter the oncoming alpha strike. The most widely used trick to bolster this early pressure is applying the Alpha-level Protector Protocol for a massive increase to their Ballistic Skill, ensuring that initial volley is precise and reliable. Other more circumstantial uses involve combining them with a forward-roving death-star to try and set-up a combo-charge and severely cripple the opposition with the Rad-Saturation effect if Vanguard are in play, while using them as foot-sloggers is also not a terrible idea as they can survive token resistance and decimate whatever comes into their wide threat-range - this tactic is best suited to the Rangers out of the two squads. Using even the standard maniple detachment - if you haven't guessed by now, the codex cannot be legally run in a Combined Arms detachment - the Skitarii can rather quickly close the gap between the opposing deployment zones, but their lack of real defence against AP4 weaponry other than that minor Feel No Pain (6+) roll should encourage the liberal application of cover wherever possible. Be wary of anything that uses Heavy Flamer or other similarly destructive template weapons as they are virtually a hard counter to your Skitarii infantry.
Thank you all for reading this article, I hope you enjoyed it! I've really enjoyed the usage of Skitarii and have found anecdotal evidence to favor both units with perhaps the Vanguard being the best suited to common applications of Skitarii forces with their lower cost also taken into consideration. Whether you agree or disagree, I would love to read your opinion on the matter and what you think of Skitarii squads. Thanks again!
I have fought as a God fights. I am Imperius Dictatio. Kneel before me and beg for your lives!
- Ervin Hekate, Princeps of the Imperius Dictatio.