Which Xbox 360 Game Should I Get Next? Please HELP!

[cloudzero2006]

i've just completed call of duty 4, and was thinking about selling it to get another game, and yes, i have thought about the online play, yes its great, but im not too fussed with the online mode much. Sooooo many class games have been released recently and i just dont know which one 2 get! Here are the ones ive been looking at, please give any personal opinions or advice of whats the best one, thanks

Mass Effect
Assassins Creed
PGR4
Bioshock
Need For Speed Prostreet (Shit reviews apparently, disappointing)

Halo 3 i can borrow off my brother so i aint gonna buy that.
If you think I have missed any, please say

Thanks for your help.