Alien Hominid (console version). C'mon, you gotta admit it's underrated...
Einhander: an incredibly badass side-scrolling shooter for the Playstation. Few people have even heard of it, and those of you who have and played it? I will blow you for a copy.
Zone of the Enders 2: pretty cool story, even better gameplay. Came short in play-length, though.
The Armored Core series (All of them except for Nine Breaker for PS2 and that shitty PSP version): endless customization. My friends and I still build Cores in AC4A and split screen that shit. We've even made "Blader" cores which are super-fast and, as the name suggests, only have blades, and we'll turn those matches into drinking games since it's completely based on skill.
Worms Armageddon: endless fun...I could play that game for days on end.
Shadow Warrior (3D Realms PC game). Like Duke Nukem with a samurai sword and shurikens.
Carmageddon: Senseless racing/demolition derby with added bonus's for it's gore, running over pedestrains and causing wanton destruction...the first two were the best.
Battlestation Midway: very complex and intelligently done.
The entire Guilty Gear series: phenomenal music for a phenomenal 2D fighting game.
and Homeworld 2... Absolutely brilliant space-themed strategy game with spherical, three-dimensional maps and massive fleets of ships at your disposal. I'M STILL trying to buy the full version of this game...and I wish they would make a sequel...