I played the first online recently. Getting way higher K/Ds than I get in this game (just a coincidence lololol) against players of similar skill levels.
I've always meant to try the second and third ones. I'm particularly interested in 3, as its a Treyarch game before they went insane with the development of this game, and I hear that it also introduced sprinting.
I mainly play WaW now that I have everything diamonded and max-prestiged in this game, as I have no motivation to play it unless I have a party of 5+ going. A bit of COD4 and BO1 on the side as well.