I spent a ridiculous amount of time playing GTA2 back in the day. I never got bored of baiting cops into chases all across the map. It was still so new at the time, and it's relative simplicity is still charming.
The greatest for me, however, are probably the two Zelda games on N64. Even when I wasn't working on the actual quest, I would just ride Epona across the field for ages, to visit Lon Lon Ranch for a race, or to relax at the fishing pond, or just to watch the scenery roll by. That huge-feeling world, in full 3D, nearly 20 years ago now...it was staggering. It was a quantum leap, and little else has ever come close since.