From my experiences playing with 1-3 other people:
Diablo 2 is always good for a co-op playthrough. There's even a storyline, although I can't comment much on it, as we skipped most of it, cause it was expositioned through NPC monologues in town.
Dawn of War 1 with the Firestorm over Kronus mod was fun to play against the harder AIs in skirmishes for a while. Sometimes even the AI won, although usually when they played Eldar, as those were really overpowered in FoK.
SWAT4 was fun to play and had a few funny situations, although it was also pretty hard (in a good way) and required patience.
LOTR: Conquest was pretty mediocre in itself, but it generated quite a few funny situations, mostly related to how the Witch King, Elrond, Gandalf and Sauron died in them.
Resident Evil 5 is pretty good for co-op. Had one or two laughs, but mostly it was alright to play. Worst part was the cutscene QTEs where both players had to press the right button, else they failed and had to try again.
And while I understand you don't want to touch NWN2 due to the interface (normally I wouldn't want to touch it due to D&D), we found the game hillarious due to the writing. Normally it was pretty bad in itself, but in co-op it got even worse with situations where the game expected the player character to be alone, yet there behind him was always the co-op player, standing around looking disinterested with everything.