Command and Conquer: Red Alert 2 (PC)
The C&C games are a joke nowadays, but this was the first RTS I played and I got really into it. I think I was able to take on 7 Brutal opponents at my peak.
Counter-Strike 1.5/1.6/Source (PC)
Thinking about 1.5 makes me sad because I used to have a 5-digit steam account for which I completely forgot the login info. Sigh. Anyway, this is probably the only game from the 90's that I still regularly play. It would be #2 or 3, tied with RCT, as my favorite game ever. I'm excited to see that Valve is finally showing Source some love with a new beta, even if they've barely changed anything.
Super Smash Bros., Super Smash Bros. Melee (N64/GC)
This would be game #4 on my list. It got me into a lot of fights with my good friends, but we always reconciled after a day and resumed beating each other to a pulp. Marth 4 lyfe