Each of the Call of Duty games through the first half of the franchise had a solid aspect that I really enjoyed, but I had to stop buying them after BO2.
COD4 MW1: In terms of the storyline and online play this one was my all-time favorite. The simplicity of the 3-5-7 killstreaks and the perks + equipment made this game the most even playing field, imo.
COD:WaW - The single player campaign was incredible, the online was fun, but not great. The 3-5-7 killstreaks still worked well, but the vehicles were whack, IMO. The zombies was the huge selling point in this one for me.
COD MW2 - The campaign was solid, but the multi-player was a mess, imo. The perks were cool, and some of the new weapons were enjoyable, but the kill-streak system was very unbalanced.
COD:BO - Black Ops multi-player was a bit more enjoyable, I thought, my favorite part of the multi-player was the wager matches. Sharp-shooter and Sticks & Stones were a ton of fun, in my opinion.
I couldn't get into MW3 and I decided to stop buying them at this point. I've heard good things about WW2, and may consider buying that, but I'm really not sure if it's worth the money for me.