It may be a bad movie, but Tron is one of the best science fiction films of all time. Stunningly creative, from the premise to the production to the fantastic Wendy Carlos score. Sure the plot may be awful, but it's so beautiful! Trivia: it was made with essentially zero digital effects or computer graphics.
Update: Dan corrects me - there are many computer graphics bits in the films, and in fact it was one of the first features with lots of CG. The central look of the film (lighting, characters, sets, etc) are all traditional film techniques, but many of the details in the computer world (light cycles, recognizers, etc) are rendered.
Tron is an important waystation in cyberpunk, borrowing heavily from Metropolis in its milieu and design (particularly the character makeup!) and The Wizard of Oz for the fantasy story. In turn it has inspired films such as the animation in Johnny Mnemonic and the navigational scenes in The Second Renaissance (The Animatrix). The cast is surprisingly good, too: David Warner, Jeff Bridges, Bruce Boxleitner, even Barnard Hughes is fun as that guy.
The new Tron 2.0 PC game is a pleasure. There was talk of a Tron movie sequel but now the rumour is there is no film. Probabaly just as well.