Skyray Missile Gunship
Oh, the Skyray.
Sweet, beautiful, under-appreciated Skyray.
It will never be as common as the Hammerhead, or Broadside teams, nor will it ever be as celebrated, but yet it remains there, acting as the favorite Heavy Support Choice of Ninja and Infiltration Tau strategies everywhere. Notable mostly for its rather unique (and useful!) rules behind its main weapon, a battery of Smart Missiles, it is a fairly versatile weapon platform. Unfortunately, it's pricey, and takes up the oh-so-essential heavy support choice slot in a given Tau Army - which, unless the army in question is either heavily focused on mobility or has more than the usual ass-load of Markerlights in it, generally isn't a nice thing.
Still, it's a fun vehicle and genuinely useful if used correctly, so it gets a nod here.
Tabletop
The Skyray itself is a Gunship - a classification the Tau have also given to the Hammerhead, since it's designed to be a support unit. It boasts surprisingly heavy armor (for a Tau vehicle), and has the natural ability of Tau vehicles to pack on a Field and get a cover save. What truly sets it apart from other vehicles, however, is that its main weapon - Smart Missiles - don't quite work like any other weapon in 40K.
Fans of the Imperial Guard have a love-hate relationship with the Hunter-Killer Missile. Whilst it can give a vehicle first-strike punch due to its unlimited range, it usually costs too much and is too wonky to be considered a serious anti-vehicle option. The Skyray gets around this problem by means of simply being a carrier for a large number of the Tau's nifty-swell precision-guided missiles - and is noteworthy in that it is not the source of its own missile attacks (usually).
See, when Tau Pathfinders or the like Markerlight something, the markerlight counters are normally spendable on improving a unit's BS, or reducing an enemy's cover save. If you have a Skyray, you can spend a point launching one of the Skyray's Smart Missiles at it - these missiles not only boast unlimited range and firepower roughly equal to a Hunter-Killer missile, but it doesn't even need line-of-sight to drop a missile strike. In other words, the Skyray can provide long-range support and target multiple simultaneous missile strikes as long as it has Markerlight support. It's particularly effective at butt-fucking Tyranid monstrous creatures and light or medium armor that otherwise thought that a seemingly-helpless unit with a Markerlight had access to a cruise missile from a support unit nearby.
In a pinch, the Skyray has 2 markerlights of its own - and yes, it can lock and fire its own missiles in a pinch. If that's not enough, it also can pack burst cannons, drones, or missile pods (the latter widely being acknowledged as the better option) to deal with infantry units or heavy infantry, respectively. With its solid armor, the Skyray won't suffer for the change of pace, since no matter what action it takes - shooting its main weapons, dropping Markerlights, or even parked in your deployment zone acting as a fuck-bus for a very bored Shas'ui - it's going to get shit done, /tg/ style. And most likely, not during its own firing phase.
But Then, Hammerheads
"But wait!" Yells the concerned non-Tau player, perplexed by this article. "This unit sounds awesome. Why isn't it used that much?"
The answer is simple: Railguns.
The Tau Hammerhead Gunship and Broadside Battle Suit are both heavy support choices, both have the awesome S10 AP1 that is Railguns, and Railguns will never disappoint as an antivehicle option - whereas the Skyray's missiles are S8 AP3 - great for busting open enemy Crisis Suits (lol) and the errant Carnifex, but horrible against the big scary AV14 Armor that tends to hit the table when fighting Space Marines, Chaos Space Marines, or the Imperial Guard. The Skyray's deficiencies are especially notable on more conventional Tau army lists; whereas one can take a Hammerhead on virtually any mission without becoming a liability, a Skyray can be mighty difficult to justify when its lowered performance against Predator Tanks and Leman Russ Battle Tanks causes glancing hits as opposed to penetrating ones (though the Skyray is just as durable as more conventional tanks, if not more so).
Even though the Skyray can provide multiple support strikes in the same turn, it's simply not as efficient; heck - the Hammerhead can double as an anti-infantry weapon too, since it has the submunition mode. No matter how boss the Skyray is (and it's very, very good), it will never be able to hold a candle to the undiluted armor-rape that is Broadside/Hammerhead Railguns. Because of these - and the fact that the Skyray isn't even that much cheaper than the other choices - it's simply an underutilized unit.
On the other hand, the Skyray does have a few uses on the battlefield. It works very well with mechanized Tau and Battlesuit-heavy lists (esp. Ninja Tau), since these are less-likely to have oh-so-precious Markerlights, and a number of Tau Armies field one to back up their Broadsides and/or Hammerhead - since it's kind of hard to argue that Smart Missiles aren't fun as hell to use. Armies which use fucktons of Markerlights frequently have several, since one critical advantage the Skyray has over the Hammerhead is that it can fire numerous times in one turn if the Markerlights allow for it.