I would go with Lost Planet as a top buy, really good single player (despite the worst voice acting and overall storyline ever), the bosses and levels are so well done. And then in multiplayer it gets better, "Lost Coast" is one of my favorite multiplayer maps in any shooter type game ever. Also the downloadable content is actually worth the price.
After that, it kinda goes into a gray area of just kinda waiting for the next bunch of games coming out. As it seems this whole past few weeks has been nothing but announcements of games coming out, with very few games coming out right right now.
Rent, and I really mean rent cause its a fun game, Crackdown, just make sure not to buy it, as you will be done with everything in the game within a day.
Make a Japanese account and download the Eternal Sonata demo (before MS starts really cracking down on this crap)
If you really like Tetris, the 360 Tetris Evolution is actually not bad (and only $30 retail brand new, so you know you can find it cheaper somewhere)