I want to vote World at War as it was amazing, but I got to go with CoD4 just for the sheer impact it had on the industry, and the quality of all of the maps. There was a consistently high quality of design throughout virtually all of the maps, and that just wasn't the case with World at War. Also I feel like the MP40 in WaW was dramatically OP, while CoD4 didn't really have any broken weapons.
But really it's about the maps. CoD4 launched with 16 maps. Shipment was a super small, basically meme map for pure chaos, but I'd rate all other 15 maps at least a 7 out of 10, with most getting 9's and 10's. Meanwhile WaW launched with 12 or 13 and I'd say maybe 2 or 3 were higher than a 7. It had a lot of DLC maps, 9 IIRC, I only remember two of them being any good, Banzai and the submarine base one.
It jumped the shark with MW2. Became less about guns, became all about killstreaks, camping choke points, and broken ass gameplay design.