MW2 if you have Xbox Live Gold. It's split between SP, Co-op, and MP, and excels in each category.
L4D2 if you like killing zombies and have Xbox Live Gold. It's mainly Co-op and MP, what with the boring AI and Versus and Campaign. Single-Player will be pretty drab.
ODST if you have Xbox Live Gold and you didn't buy the extra map packs for H3 and you enjoyed H3. It's sorta short, but has more maps and stuff, so you can get back into playing the MP. (I couldn't play more than half of the playlists because I didn't have the maps, but after I got ODST, MP was sorta fun again. Once again, buy this if you don't have the map packs.)
AC2 if you don't have Xbox Live Gold. Lengthy Single-Player campaign, and some extra missions in the form of DLC coming soon.