1) Freespace 2
Great, great game. Still going strong ~eleven years after it's release. Seriously, is there anything better than seeing two massive space ships slicing each other in half with gigantic beam cannons? (Immah chargin' mah lazer!)
2) Max Payne
The story, the action, THE BULLET-TIME. My favorite shooter game. So many awesome one-liners. Too bad the movie was absolutely HORRIBLE.
3) Command & Conquer: Red Alert & Tiberian Sun
Can't decide which one is better, Red Alert or Tiberian Sun. These games introduced me to Real Time Strategy. Lots of good memories from these two games. Still have a copy of Tiberian Sun lying around somewhere...
4) Mass Effect
My opinion: ME1 and ME2 were both just as good. ME2 had better characters and combat, ME1 had a better overall story. Yes, there are flaws in both games and neither one is perfect, but they sure are better than most of the more recent games. Just my opinion though...
5) Tekken 3
A blast from the past. My favorite game ~10-11 years ago. Heihachi was so damn frustrating though... Still play it from time to time when there's nothing else to do.
Fun fact: To this day, I have NOT been able to unlock all the characters.. I'm missing ONE.
6) Crash Team Racing

Lots of good memories from this one. Used to play it all the time as a kid. Beat my dad at it quite a few times too

All the other Crash Bandicoot games are really good too, but this one is my favorite.
Funny how most of the games on my list are ~10 years old or even older.
I didn't list all of my favorites, because I thought it would be better if I just mentioned one shooter, one rpg etc.