This list of my favorites is in no particular order.

GEOMETRY WARS 2 - A great game that improved upon the stellar original. I love all the different game modes.
MARBLE BLAST ULTRA - playing with 8 people in an online match is a blast! I really enjoyed the single player as well.
N+ - Such a simple premise of a game yet so damn fun.
PEGGLE - Who knew playing a game resembling Klinko from the Price is Right while you play as a blue unicorn could be so addictive.
WIK: FABLE OF SOULS - A game not as popular as the others but very fun regardless.

Castle Crashers was my first Arcade game i bought and i enjoyed it.

Battlefield 1943 is teh best atm

I really liked both Battlefield (Would be even better with a few more maps) and Castle Crashers too.

The Behemoth, the company that made Castle Crashers, and Alien Hominid has another game coming to XBLA in 2010. It has the same graphic style of Castle Crashers and I'm sure it's going to be a great game.

If you want to see the trailer for the game click on the link below.

http://www.1up.com/do/newsStory?cId=3173301

Looks like a fun co-op game

There are quite a few I like... Castle Crashers, The Dishwasher is pretty fun. I really actually enjoyed playing Wik quite a bit too.

The new battlefield 1943 is really good

Geometry Wars 2 (It felt good to finally get that smile achievement), Ikaruga, Splosion Man are some of my favorites. I'm starting to really get into castle crashers.