As someone who has primarily (read: almost exclusively) played console games, this is a bit of a tougher task for me. I grew up watching my dad play Unreal and Quake on his PC, and I played a little of the latter (and some other games) on there myself. I've started buying a good chunk of games for my PC now (given the Holiday Sale and all), and I'm planning to get something closer to a 50-50 split on my gaming medium now. That said, this would probably be my 4:
1. Urban Terror: IT started as a Quake III Team Arena mod, but it's the only part of that game I actually played. I still say things like the variable breath for jumping (based on remaining health) and the need to bandage at times are still some of my favorite things a shooter has ever done.
2. Unreal Tournament/Unreal Tournament 2003: I don't remember which I watched my dad play, but I loved it, and it's why I got Unreal Championship as my first Xbox LIVE game. It's also why I was SUPER disappointed in the Xbox LIVE community when it didn't flock to Quake 4 and Unreal Tournament III on Xbox 360 like I wanted them to, because it meant nearly-barren servers I could never play games on.
3. World of Warcraft: Didn't actually touch this until I got to college in 2008 (roommate showed me it), and I never played it in long stretched (4-6 months, with breaks of roughly 6 months). I don't play it now, but I'm still impressed at how it can suck your time away through many different things (raiding, PvP, or even something like Pet Battles), and the fact that it's considered to be in a terrible state at 7 million+ members really says a lot about a game that was peaking around 12 million paid subscribers, given the fact that almost every alternative is free-to-play.
4. Portal/Portal2: I'm still impressed by the hilarity of these games. I love Cave Johnson, loved the way you get around the levels, loved everything about the series.