Honestly, WaW is the CoD I have the best memories from. But that's mostly because it was my first online CoD, and it was where I discovered how awesome online fps is. Also, I really like WW2 games.
But, if you ignore the 1MA and quickscoping in MW2, then that's actually my favourite. I just think the gameplay was really smooth and had a good flow...if that makes sense.
CoD 4 was ground breaking, and it was and still is a huge title. But I started playing it 4 or 5 months after WaW was released, so I haven't had enough time with CoD 4 for it to be my favourite.
I have to say though, that Black Ops is actually getting more and more fun for me. I know alot of people hate it, but I've never really had any problems with it. The main reason is that I think the weapons are really well balanced....except the Famas. But that's the only gun I have a problem with. I hear people complain about Galil, AK47, Commando, and of course AK74u. My opinion is that those weapons are just overused, not overpowered...