If you can somehow find a way of acquiring it and making it play, I have many fond memories of doing co-op with my dad in
Hexen 2. I might as well throw in my endorsement for Diablos I and II for somewhat the same reason.
Terraria is more of a build-em-up than anything straightforward, but it does have some interesting bossfights and dungeon delving for the cooperative player. It's far,
far more slow-paced however, as most of your time is spent digging, building, decorating and so on.
Icewind Dale/Baldur's Gate and their ilk are also, really, not for the faint of heart. Very slow-paced, very complex, and things tend to take a long time. If you're in for a bit of a gaming haul it can be excellent fun to play with a group of friends, but from the sounds of it it's not what you're really looking for.
Titan's Quest is a Diablo-like, but I personally didn't care for it quite as much... The skill/class building is even more unforgiving than Diablo 2, where you can very easily build yourself into a corner where you'll be ineffective against anything and everything from then on out. You're basically forced to pick one of the few approved skill builds in order to take on much of anything. Also, the action is slower-paced than Diablo 2.
There's a niggling sensation in the back of my skull that I know about some other decent right-in-the-action co-op games (seeing as that's generally what my dad would play), but I can't quite seem to think of any right offhand... I'll make an appearance later if it comes back to me.