1)Fallout:
The game that really kindled my love of PC gaming. This superb rpg will always be the golden number one for me.

2)Fallout 2:
Continuing on the brilliant premise set by the original, this one expanded everything from gameplay choices to storyline. RIP BlackIsle.

3)Tribes 2:
My first online fps, I got really into one of the custom mods called 'Heavy Metal'

4)Star Wars Rise in Power (MUD):
All MMOs are based on a MUD skeleton. The difference is that modern MMOs are limited to what they can display on the screen whereas you were able to do so many more amazing things in a text based game since it relied on your IMAGINATION.

5)Any game made by BlackIsle or the companies derived from (Trokia, Obsidian for Arcanum, Vampire Bloodlines, Planescape, KotR, ect...)


Honorable mentions:
TF2
Warcraft 2
Warcraft 3 custom games (DotA classic, DotA allstars, Sheep Tag)
Diablo 2
Vanguard City of Heroes - This MMO had amazing potential but failed horribly
Duke Nukem 3d
Dungeon Keeper