Re: Best multiplayer/2 player games, online and offline
I don't know about the last 4-5 years.
Other than what has already been mentioned: Portal 2, L4D series, Monaco, Payday series, Risk of Rain, Diablo 2.
imo, Portal 2 is the best true co-op game ever. Some of the other games are like two players playing two single player modes on split screen. Portal was designed for co-op.
L4D and Payday might fit what you are looking for, tho.
Risk of Rain and Monaco (latter also made for co-op; ideally a full 4 player room) are stupidly fun indie games.
Diablo is Diablo.
Then there's always 2K and Fifa. Co-op is always fun in those if you are playing with someone who knows what they are doing.