hmm well I would say Halo 3 since I still play it after buying it over 2 years ago, where as Call of Duty: World at War while having somehow more entertaining multiplayer then its predecessor, it still felt too much like a re-themed Cod4. I rented Cod5 and sent it back about a week or 2 later.
Cod 5, the main reason is that i own a PS3 and don't own a Xbox, but also because of the Zombie mode and my preference of a more 'realistic' multiplayer.
cod 5 is not very good coz the graphics suk and its not a very good storyline but halo has new releases all the time such as halo reach and has a good storyline coz the halo games all link together and the cod series is all random people from world war 2 or the iraq war and modern warfare is betta then cod 5 sooooo im gunna have to say halo