i like different ones for different reasons...
cod 2 was cool, because is was a launch title for 360 and those graphics were amazing, because we had never seen anything like that before...
i like cod 4 because the multiplayer was such a unique experience... it was so solid, and i think that everyone loves cod 4 because it was the first cod that did MP right...
i like black ops, but i'm not sure why i'm so addicted to it... it's a good game, and i'm not sure if i like that you have to buy your upgrades, but i've been playing the game since it came out, and i thoroughly enjoy it