Black Ops was fun, and I had most fun with the wager match gamemodes.
It also has zombiesssss
However, MW3 has gun game, one in the chamber too so personally i'm content with that. It also has some other really fun, new gamemodes.
MW3 does have a stuff ton of DLC now though and I don't know how much of it is free because I was gifted the Hardened Edition (so I get everything free with Elite Premium).
I'd go for MW3 personally but zombies could be a deal breaker for a lot of people.