I also polled the rest of the staff and found a somewhat common thread: quick arcade-style games with simple mechanics that allow for anybody to drop in and have a good time.
Nick, Locke and Marc all chimed in with good examples:
Nick: "Smash T.V. and the Raiden series are ones I always return to. I think the arcade style games tend to hold up well over the years and don't require as much of a time investment, so it's perfect to rekindle a co-op romance."
Locke: "I really like the old beat em up games like Streets of Rage, Golden Axe, and such. They are nice to just hop in and play. If you have a larger chunk of time, toss some RPG elements on them and give me some Baldur's Gate Dark Alliance, Dungeon Siege, or something along those lines."
Marc: "Arcade style co-op is something we go back to over and over: Castle Crashers, Geometry Wars, and pretty much any of the retro remakes that have been so popular the past few years. Short time commitments and no learning curve to speak of, perfect for any occasion."
Of course, not everybody returns to the golden age of the arcade for their co-op fix, as John and Jason are quick to point out:
John: "For once, I won't talk about a game that was released more than 20 years ago! Sol Survivor is a tower defense game released in 2009 that I've gone back to a number of times. On the surface it's pretty much an ordinary TD game with a smooth 3D camera system and some nice co-op modes. When you jump in with other players you very quickly realize how important it is to work together. You don't have access to all of the game's towers and abilities. Instead, you pick a character with themed units (fast and shooty, defensive, tactical, etc.) and go into battle with just that loadout. This pulls some of the strategy out of the levels themselves and encourages players to choose complementary abilities to ensure everyone's survival. It's very well-balanced and has just the right amount of challenge to keep me playing even five years down the road."
Jason: "Secret of Mana - Still one of the greatest co-op experiences in my book, especially for couch co-op. Action RPG where the second (or third with that funky controller extend pack) player gets full control over their character and can equip them however they see fit. Plus, all characters are involved in the overall story so everyone feels like they're a part of what's going on."
So there you have it - games that are either beloved or mostly forgotten, but still provide great co-op experiences that you can return to time and again. Now I'm turning it over to you, dear reader. What games do you find yourself going back to?