Call of Duty The Big Red One: This was my first CoD game, and I loved it to bits and used to always play it. It got me interested in the series. The single player was amazing.
Call of Duty 3: I don't remember much at all about this, I did have it and used to play it, but for some reason just can't remember much about it.
Call of Duty 4 Modern Warfare: I could play the single player over and over again, had loads of fun with friends on my first multiplayer CoD game, such good fun to play and the most memorable CoD I have had.
Call of Duty World at War: I liked the single player challenge a lot, and was super pleased with myself when I finally completed it. Didn't like the multiplayer too much so mostly went back to CoD4. But the zombies was so much fun and I loved it, and played it all the time.
Modern Warfare 2: Both the single player and the multiplayer were great to play and lots of fun to mess around in. Played it loads and loved the campaign as well as the multiplayer. Spec Ops was also quite good for a challenge.
Call of Duty Black Ops: The single player was the most interesting in the whole series, by far, and the multiplayer to match. I do so much better on Black Ops than any other CoD game so far. And all the new features are brilliant. Also the return of zombies is excellent, and I play it more than I did on World at War. I play it all the time.
In conclusion I just can't really decide between a few of them, but at a push I would have to say Black Ops. It has only been out for a few weeks but first impressions are everything, and I love it!