Hmmm, okay here goes :
Crackdown/riotact - especially if you can find someone to play it with, starts off slow, but picks up drastically once you start getting your levels up.
Chromehounds - absolutely awful single player campaign, but online in a mech team it's pretty great. Still my favourite game of the new generation.
Rainbow 6 Vegas - i guess Vegas 2 now that most of the bugs have been ironed out and the AI has been fixed. Great multiplayer fun (Terrorist hunt), doesn't get old, but the level/ace system could have been much more involved.
EDF - hit and miss per level, but some great moments throughout. Absolutely zero regard for frame rate ends up with screens packed full of enemies and single digit frame rates. pretty fun. Terrible achievements
Halo 3- i have to admit this was a huge disappointment, but it's still worth owning for the coop online play, which is great fun.
Just Cause - if you can pick it up budget, i had a lot of fun with this sandbox game, mainly from just flying around.
Rez HD : ... it's Rez in HD. err. What's not to like?
Braid - ignore the internet arguments, it's a clever puzzler with some interesting situations. It's not a platformer, don't be fooled by the screenshots.
Geometry Wars 2 - fantastic shooter, good fun playing the
Castle Crashers - slash em up from the Alien Hominid team. Some online issues, but great fun.
Puzzle Quest - bejeweled mini rpg, good fun.
If you don't have a decent PC :
Orange box - can't really go wrong with teamfortress, HL2, Hl2 ep 1, hl2 ep 2 and the utterly fantastic Portal. Worth it just to play Portal alone.
Mass Effect - technically up and down with some horrific moments of texture loading at times, but a great story, some great interactions, excellent music.
COD4 - took the shine off H3 online for me. Single player is a bit too scripted though especially compared to Halo 3's AI driven action. Online is great though, good levelling system, some nice ideas etc.