There hasn't been a year since the late 90's where I haven't re-played Doom II/Final Doom at least partially. I'm really good at all of those games by now
I've also re-played Shadow Warrior and Rise of the Triad a lot (the 90's originals of course).
Otherwise, just about any good fighting game or arcade shooter is fun to return to whenever I have half an hour to kill. I used to play Street Fighter Alpha 3 every time I did my laundry.
I can always play 5 minutes of Stunts or make a new race track. I still have tracks that I've built all the way back in the 90's, but I keep adding new ones every year.
