Paralegal Hell covered this topic and I just had to put my two cents in. Here's a short list of movies that I have to watch if they show up on TV, in no particular order.
1. The Ten Commandments (1956) - Charlton Heston, Yul Brynner, Edward G. Robinson. Directed by the legendary Cecil B. DeMille. What's not to love? One of the first, and still the best, grand epic movies. The only problem is they show it just once a year during Passover. (This very weekend, I believe.)
2. The Hunt for Red October (1990) - Sean Connery, Tim Curry, James Earl Jones, Scott Glenn, Sam Neill, Alec Baldwin (pre-crazy) and Fred Thompson (pre-Senate). The cast alone makes this movie worthwhile. It's also the only Tom Clancy screen adaptation that didn't suck. Admittedly it's a lot different from the book, but if you tried making it true to the book you'd end up with somthing about as long as Lord of the Rings.
3. UHF (1989) - Weird Al's completely over-the-top comedic cult masterpiece. Two of the key actors(Michael Richards, Fran Drescher) went on to be huge 1990's sitcom stars, but were largely unknown at the time. It would have been a much bigger hit had it not been released the same unforgettable summer as Jurassic Park, Batman, Bill & Ted's Excellent Adventure, Indiana Jones and the Last Crusade, Major League, the list goes on.
4. Casablanca (1942) - If you haven't watched this movie and loved it, YOU'RE A COMMIE!!! The entire movie is a masterpiece that's been much emulated over the years, and marks filmmaking's transition into a true art form. I refuse to buy the DVD just so I can be caught pleasantly off-guard on the rare occasions TCM pulls it out.
5. Speed (1994) - Dennis Hopper in perhaps his best bad-guy performance, along with some other people. Every time you think it's going to end, it just keeps going. Very underrated movie.
6. Demolition Man (1993) - Sylvester Stallone, Wesley Snipes, Denis Leary AND time-travel to the future! How could you go wrong? So over-the-top awful you just can't turn away. Like Thanksgiving with your in-laws.
7. Necessary Roughness (1991) - Scott Bakula, Hector Elizondo, Robert Loggia and (regrettably) Sinbad. A college football team so bad, they strive to win just one game all season. Rob Schneider in quite possibly his funniest role ever as the play-by-play announcer.