The Crysis of 1998, one of the most revolutionary FPS in the history of gaming itself.
The environments are vast, varied, lush, and breathtaking. The textures are much larger and more detailed, colorful, and varied than those in Quake II or Half Life (both extremely popular in the same time frame as Unreal), resulting in a much more interesting exploratory experience as you delve through the variety of levels-from dark, dank caves to huge outdoor settings with kaleidoscopes of bright colors.
The game requires a certain amount of strategy to complete, and the plot is actually much more complex then at first look, but you only know it a step at a time, which makes the game more exciting to complete.
The weapons are fairly well balanced and interesting in single-play, the game's AI is good, enemies are detailed and tough to kill.
You can't call yourself a gamer if you haven't play Unreal yet, its a great experience and still holds up very well. Buy it, its probably dirt cheap by now.