Personally for me, I would have to say call of duty modern warfare 2. I've always liked it since it came out. It has a great online play and very fun. Black ops though. It's good but I still prefer MW2. I'll just have to get use to black ops before I actually like it.
Halo's storyline on one player has gotten really boring for me. I liked it with 1 & 2 but the new ones aren't that good for me. The online gameplay is the only reason that I will play it.