.

PGR 4, nobody mentioned, best Xbox title.

State of Decay
Forza Horizon
Forza 4
PGR3
Shadow Complex
Catan (don't confuse it with the shitty, shitty PSN version)
Battleblock Theater
Trials HD
Trials Evolution
Left 4 Dead 2
Super Meat Boy
N+
Mark of the Ninja
Witcher 2
Geometry Wars: Retro Evolved
Kameo
Dust
'Splosion Man
Crackdown
Crackdown 2
Lost Odyssey
Blue Dragon
Splinter Cell Conviction
Perfect Dark XBLA
Guardian Heroes
Bastion
Halo Wars
BK: Nuts and Bolts

I know I am missing some XBLA gems but here is a good start for you.

+1 Not my favorite 360 title but my favourite  360 racer!!! Very underrated Graphically also.

Dead Rising
Halo 3
Forza 4

Theres also quite a few PC/X360 games such as: Alan Wake, Metro 2033, Witcher 2

Did anyone mention Dead Rising? Cause that's a great title also!

Full-house poker is pretty fun too. :3

Hmm, you've already covered like half the good exclusives it seems ha, even more if you include PC. But I guess I would add...

Dead Rising
Tales of Vesperia
Shadow Complex
Geometry Wars 2

Do you have a PS3? To be honest, about 99% of the PS3 owners I know only play third party games and don't own a single first party title. The 360 is in a similar position except most of the people I know own Gears and Halo. Third party titles seem to be the reason to own the HD Twins. You'll notice a few things the 360 does better than the PS3 and a few things that it does worse but if you have one, you really don't need the other unless there's something you're just dying to play. That doesn't seem to be the case since you want people to tell you what to buy. I say save your money, even if it's just a little bit of money.

Lost Odyssey.