Mass Effect by far.
The reason being it's exactly the kind of game I've always longed to play. Not to mention the voice acting, graphics, gameplay and (most importantly) the characterization, were top notch. Roll on M.E.2! Left 4 Dead is in there too - as I'm a big Zombie fan, but not so much of the old-school variety - I like my undead to run!

Crackdown was fantastic fun to play (still is) and on the original Xbox, I thought Max Payne, Fahrenheit (a.k.a. Indigo Prophecy), Deus Ex: Invisible War, Syberia II, Dreamfall: The Longest Journey and Call of Cthulhu http://www.callofcthulhu.com - the latter being pretty unbeatable for atmosphere and general all-round creepiness in a game on that platform I reckon.
The original 2 or 3 Tomb Raider games on the PS1 before they started to suck, Corporation, Another World, ToeJam and Earl, Earthworm Jim and Sonic The Hedgehog on the Sega Megadrive. Oh, and Shadows of The Empire on the N64 - I thought that rocked at the time.