For me, Sub Rebellion on the PS2. A game so unheard of it doesnt even have a wikipedia page.
The game takes place in a near future world where (if memory serves) water levels have risen to an altitude of about 1500ft due to a series of earthquakes that apparently must have destroyed all of Antartica to cause something like that. Humans (natually) adapt to this by biulding underwater complexes from what little land remains above the water. Due to the calamity in question most countries were completely wiped out by the rising water, and what is there for the survivors to do? PICK SIDES AND FIGHT OVER WHATS LEFT (natrually).
Now most of the world being waterlogged to a new definiton of the word warfare had to undergo some drastic changes to keep it going, and the main form of combat is obvious from the title, submarines.
The game has you piloting a new prototype combat minisub that in its very stride bends and breaks physics around it to store infinite torpedoes for sub-to-sub combat, a 'needle gun' which i believe is a scaled down railgun for close combat and in between torpedo reloads, a retractable missile launcher and machine gun for engaging ground and air targets, and a whacking powerful nuclear engine to propell this about-shipping-container-sized minisub through water at speeds of about 163 m/s.
Apparently memory does serve...
Anyway at the stage where you start the game there are pretty much only 2 factions remaining alongside some chaotic neutrals that may be mentioned later. The side youre fighting is the much larger, superior, eviler Empire who control most of the world and are using most of it to find and hunt down your side (more on that later). Thier arsenal incuding everying from the equivalent to underwater TIE Fighters to... and i wish i was kidding, underwarer Star Destroyers and other Irregulars.
Your side is the aptly named Alliance aiming to stop the Empire for no good reason as i saw it while facing near impossible odds silimar to the afore mentioned Sea Destroyers or as theyre called in the game, Daglezenair-class 'oh shits' (oh shits isnt true but you would when you see one open its bow to reveal a 20-tube torpedo launching array) with nothing but some severely outdated minisubs, no underwater capitals at all, oh and you in your micro-doomsday machine.
The two sides are surprisingly deadlocked in a battle to both kill each other and salvage ancient tech from an alien civilisation that turns out to be Atlantis (and still alien) only its spread all over the place from whats left of polar ice caps to the depths of the pacific.
Anyway in true pseudo-Star Wars style you eventually win and at the same time save the world from its drowning problems, if that even makes sense.
This game is seriously obscure, the only mentions of it on the net are on game help sites and wikipedias list of PS2 games, the developer is apparently Metro 3D but i cant tell which as ones site is down and the games official site is long dead.
Not to say its bad, its a pretty hard game but the combats thorough and everything has its use, missions and boss fights are extemely varied too and the game rewards you for exploring the large mission maps.
Holy shit, how long did i spend writing this...?