if youre getting halo 3 id get cod4 too. its better, on live and on single player. imo by a long shot.
creed is good and very long, but dont rush through it, take your time and do ALL the side quests first before finishing the assassination on that bit. it gets a little same-y after a while but its still an awesome game.
guitar hero 2 or 3 are both great games if you like music.
dead rising is possibly the best single player game ive ever played, although its got a typical zombioe infested mall storyline, theres loads of twists in it which make it great, plus its got a LOT of achievements for you to work at and its got so many ways to complete the game its just amazing.
gears, although a bit dead after halo and cocd4, is still a must have. its great, third person tactical shooter. bit of a change from the typical fps that i mainly play on live.
crackdown is fun for a while, then it starts getting boring after the get 5* in everything. although imo if you can afford it, buy it
nfs most wanted is the best nfs game ive played yet, and i love it, and its also great for 2 player too. same as crackdown for this, buy it if you can afford it
small reviews on my top games for 360
