Manta
You play Tau and are in an Apocalypse game. You need firepower. You need to get your troops into battle without them being shot up by that Reaver Battle Titan your opponent fields. You need big. You have a LOT of disposable income and incredible modeling skills. You, my dear neckbeard, need a Manta.
The Manta is one of the largest, heaviest, dakkaest and most expensive model ever made by Games Workshop, second only to a particular walking rape machine that could solo an entire apocalypse formation on its own. Sold at £1,066 (or roughly $1300 for those from Glorious Nation of Americaland) it is prohibitively expensive even by Forge World standards. Seriously, you could get yourself a good laptop, next generation console and desktop for this kind of money. It is notably the only model in the game that has a gun that can't reach the other end of the model. It is roughly as wide as two Thunderhawks are long and one and a half time as long, sports enough weapons to lay waste to entire armies and is able to carry an army on its own. Intended as a superheavy drop ship, it can pull double duty as a small spacecraft to defend Tau capital ships from enemy action and can be employed as a Titan hunter. The funny part? It's the size of a Fury interceptor. Which, canonically, Imperial carriers hold 1000-2000 of. Makes you feel sorry for the "fighters" launched by Tau starships against other warships, doesn't it? Despite that, a single Manta is equal in firepower with most other factions bomber wing, and quite capable of destroying an escort ship on its own with a lucky roll. Furthermore, it EATS enemy interceptor wings thrown at it on 4+. Makes you wonder just how low-tech imperial small space craft are if something equal in size is outperforming dozens of them by that much. Or perhaps GW is really really shitty at keeping scales consistent between different books let alone different game systems.
Interesting enough it comes with quite a few models on its own: 2 Devilfish, 2 Hammerheads, 8 Battlesuits, FOUR DOZEN seated (and therefore useless in a regular game) Fire Warriors, eight Drones, and one seated Ethereal (also useless unless you make a floating space pope throne). So you could theoretically subtract the cost of those models (between 450 and 500 dollars). The cost is still absurd.
Weaponry
The primary weapons of the Manta are a pair of Heavy Railguns, the largest weapons in the Tau arsenal. Their regular shots have as much power as the Imperial laser weapons most commonly found on Titans. While lacking a blast radius that these weapons normally have, the regular munitions of the Heavy Railgun are more than capable of laying waste to an Imperial Titan. Its second mode of fire is using Sub Munitions: while not as lethal as the regular shots they prove a very real threat to MEQ forces and can even deal significant damage to large Tyrannic Bio-Organisms.
Its secondary weapons are six long-barreled Ion Cannons. Nearly as potent as Plasma weapon and with one helluva range, they chew straight through anything that does not wear Terminator armor at a frightening speed with 3 shots per weapon.
As its tertiary weapons the Manta sports two missile launchers: one being a regular Missile Pod that is for all intents and purposes as good as an Autocannon. The second is actually a bay containing ten Seeker Missiles, allowing for a quick barrage of deadly anti-vehicle fire against enemy flyers for when its primary weapons are in use.
Finally the Manta sports no less than 16 drone-controlled long-barreled Burst Cannons all aimed in different directions. While they are the least potent of the Manta's weapons, the sheer balls-out power that these weapons provide Dakka (you will always have at least 24 shots no matter the direction the enemy comes from) and devour anything the size of a Guardsman that dares to come close. Even the largest of creatures can be taken out by the sheer magnitude of fire these weapons can pump out. A potential maximum of 96 LB BC shots a turn! Put ALL the markerlights on a manta. Do it!
Transport
The Manta is unique in that it has two separate decks that it can carry troops on: an upper one designed for Fire Warriors and Drones, and a lower one designed for vehicles and Battlesuits.
The upper deck can hold 55 models: four full squads of Fire Warriors, Breachers, Pathfinders or or even more squads of Stealsuits, six support drones and a seat for the Commander, who can only be an Ethereal (including Aun'Shi), El'Myamoto or Shadowsun (who despite her armor is not Bulky).
The lower bay can carry up to 145 models. Of these there can be 8 Battlesuits or 4 Broadsides (count as 3 and 6 each), up to 4 vehicles (Devilfish, Skyray Missile Gunships or Hammerhead Gunships, and yes you can load up the Devilfish with Fire Warriors without them costing any cargo space) that cost 30 space each. Now if you have done the math you will realise that this is a total of only 144, leaving 1 space for an additional model. See the previous paragraph for hints on how to fill that spot.
Verdict
If you field a Manta, you are automatically That Guy. Nobody else would buy, paint and field this mothermucker. While having a staggering amount of Dakka, it is hardly the most cost-effective way to get it. If you want the Titan-killing power of Heavy Railguns, field the Tiger Shark AX-1-0 instead: it costs only a third of the points and a sixth of the cash. Use the rest of the points and money elsewhere, stuff your Fire Warriors and Battlesuits in an Orca and drop them from there. Even just fielding this thing is a pain in the ass due to its HUEGness, and collectors would be better off spending money on cheaper models. The Manta is so ridiculous that in Battlefleet Gothic it gets a 4+ save because fuck you, that's why.
Scratch-built Manta
Whenever monetary resources are not available, sheer skill and enthusiasm can allow a dedicated 40k fan to get his/her own Tau manta, as shown by some awesome people who have managed to build their own mantas using foam, carboard, etc. The example shown to your right used only $40 of foam plus required tools and extra materials. Of course this is hard work, but hey! You are a smar/tg/uy! You get shit done! And because Forgeworld wouldn't let a filthy xenos aircraft be the costliest thing they ever sell, the Imperium's Warlord Titan!