Two categories I feel.....Console Games and PC Games. Please specify the genre and say why you think they're so good.
My votes are:
PC Game: Descent 1,2 and (less so) 3. Genre is a cross between flight sim and FPS. First two versons were fanatastic addictive fast-paced action shooters, challenging enough to be rewarding when you made it through, but not so challenging as to be not fun. 3 is lusher graphics and more sophisticated gameplay and storyline, but its lost some of the simple addictive formula of the first two.
Console: Gotta be Halo on the X-Box. Space-alien type FPC. Huge environment, with lush graphics. Coherent compelling story and outstanding multi-player and online options.