I think with Starcraft, Deus Ex, and Half-Life, in that order, you'll be set for a while. If you need more after that, check out Fallout 1 & 2, which ought to keep you busy for as long as you want. Warcraft 2 can be a lot of fun, too- it's a very simple RTS compared to anything newer, but setting up huge battles against AI players with the map editor is pretty exciting.