Quake(world) and
Unreal Tournament are my top FPS games ever.
Total Annihilation is an epic RTS, that is still lots of fun today.
Giant Citizen Kabuto! The game is split in three parts that play completely different (third person shooter vs monster on a rampage vs third person RTS). It has loads of humor and is a blast to play. Made by former members of Shiny Entertainment that also made
MDK (another highly recommended game).
Dungeon Keeper, or any other game made by Bullfrog

You need to build a dungeon, amass vast amounts of gold, slap your minions around, and slay all the heroes that enter your domain.
Homeworld, might a bit too recent, but that game was so freaking awesome. Its basically a RTS in 3D space. There was recently a remaster for this game that also includes Homeworld 2. Have not tried it yet though, so cannot say if its any good. Sadly the expansion Homeworld Cataclysm is not included, as the source code was lost
