I personally think MW2 is the best out of the COD series.. It was fun and most definitively challenging. The spawns also didn't suck. MW3 has terrible spawns :3 And on MW2 my clan/friends would actually plan against the other team.. now we sort of just run around with out heads between our knees..
Speaking in terms of a nice single player experience, the Half-Life series would have to take my vote. With an innovative physics engine, edge of your seat moments, fun guns, and an intriguing story line, you can't go wrong.
For a multiplayer experience, the COD series with the exception of Black Ops would get my vote. The whole gaining XP to unlock various attachments and perks keeps me coming back for more. Nice graphics and smooth responsive well mapped controls makes the whole series stand out, as well.
Even tho I've never played it, the first halo, heard it was really really good, then halo2, then 3, all my fav, went downhill for me from there, CooD now, I enjoy mw3, but my fav was definatly cod 4 by far, amazing, nothing wrong with the game, on BF, on BF3, all mazing games on my list at least.