I've had good success with Quake II and Age of Empires -- the original one.
Quake II has a co-op mode, and my (girlfriend at the time) really got into it. Then I started on Age of Empires (the 1997 game) and since we can team up, we played that plenty.
But we haven't played other games, not from lack of trying. I think the success factors for those two games:
- Simple, Easy to play
- The game eventually ends
- Did I mention Simple?
When we got bored with AoE we eventually tried AoE II. The game was much more complex than the original AoE, and a game tended to "never end" -- you'd save and continue the same game forever, whereas AoE we had it down to a 45 minute to 1 hour game.
That being said, my wife got into the Sims, and I didn't, so a gamine duo we are not. And I don't push it either.