I have always been an EA freak when it comes sports games, but after I bought my 360 2 months ago, I am sold on the 2K7 series.
Here are some of the titles I would suggest
2K7 College Hoops
Major League Baseball 2K7 (Comes out Tuesday)
EA's College Football '07
Fight Night-Round 3-This one is a classic and you can pick it up pretty cheap.
Just a heads up, Madden for the 360 SUCKS IMO (I was very dissapointed). I have Madden for the PC and it kicks any consoles ass in EVERY aspect of the game. The Madden PC community is also better setup for Franchise league play.