For me, up until MGS4, it was the MGS series. I played the hell out of MGS1-3. I also thought 4 was amazing too, I just got to the point where I was exhausted from the series. I honestly haven't even attempted to play the new MGS that came out. Normally I would be all over every new screenshot and every new trailer, but I just feel like they are overdoing the series now. I'm sure I'll play the new one at some point, but I just don't care to right now.
Other than that, I spent quite a bit of time playing WoW and also this PS2 game called Culdcept. Very unkown game, but it's a fantastic multiplayer game. It's a mix of monopoly and magic the gathering. That's the best way to describe it.