Morrowind, even without mods it's a fantastic game but mods take it to a whole new level. Tamriel Rebuilt, Distant Land generation, etc. There's so many good mods.
DOOM (and DOOM II) Still the king of the Boomer Shooters.
Return to Castle Wolfenstein, the original Wolfenstein 3D games were important but today they're age shows a lot more and DOOM is just better for a boomer shooter, however Return to Castle Wolfenstein falls in this interesting niche zone between old school Boomer Shooters and something more modern. It's a fun game that brings a lot to the table for what it is.
The original Halo is still a classic and the first real modern FPS game. Despite coming out the same year as Return to Castle Wolfenstein the mechanical superiority of Halo is quite evident, it broke new ground. Depending on how old is old to you Halo 2, Halo 3, and Halo ODST are also excellent games, though unfortunately one of the best parts of Halo 3, the Forge, is less fun today because the multiplayer scene has moved on.
Star Wars: Empire at War, the only RTS type Strategy game I ever really enjoyed. Like Morrowind its best with mods, my favorites are Awakening of the Rebellion and Fall of the Republic.
