1. Final Fantasy IX
I never actually finished this game (I'll start it up again when I have the time), but I have so many fond memories of playing it as a kid.
This game ruined my eyesight and sucked away hours, and hours of my life. For a few months or so, I literally played this game for ~7 hours a day. Sadly, I played custom (UMS) games instead of actually laddering.
3. Half-Life: Counter-Strike
My favorite shooter of all time. I've literally spent hundreds and hundreds of hours on random pubs.
4. Portal 2
It may not be better than Portal since it is a sequel, but it still manages to be a perfectly satisfying experience that just feels really good. The puzzles are challenging, but never hard enough to really trouble you and mesh really well with the absolutely stellar writing.