That one title which you most commonly tend to put on whenever you feel like playing a game.
Way back a couple of generations ago it was Jedi Academy multiplayer (who else remembers those days?). It was a ton of fun to play with friends when I wanted nothing more that to relax and have a good ol' chat.
Once Halo 2 came out it took over that role, including every single sequel (yes, I was very much a Halo fanboy. Custom mode has a habit of doing that)
That stayed all the way up to Halo:Reach, but by that point I gave up on console gaming and migrated over to PC.
A couple of MMO phases aside, Mount and Blade/Napoleonic Wars was the game me and my friends most commonly could be found playing.
I'm sad to say though, but I am now without a "go-to" game. So it would be interesting to perhaps get some ideas on games that seem to have a lot of re-playability for others.