I've not seen...a lot of things!
Tarantino's "Reservoir Dogs" or the "Kill Bills"
Anything by Sam Peckinpah
Most of Stanley Kubrick's flicks (the exceptions being 2001, The Shining, Eyes Wide Shut and Dr Strangelove)
Maltese Falcon
70s hits like Chinatown, French Connection 1 or 2, Shampoo, Sirpico, Dog Day Afternoon
Goonies (my co-workers used to give me grief about that. I actually bought the movie but just have not been able to bring myself to watch it..)
And, this may belong in another topic, but I did NOT like "Batman Begins" (or the Heath Ledger swan song "Dark Knight Rises"), "Iron Man", "Spiderman", etc. so I've avoided all of the sequels. I also was no great fan of the STAR WARS series---all though I saw the first 3 (now parts 4, 5, and 6). When "part 1" came out, it was so awful--I passed on "2 & 3". I am sort of curious to see 4-6's characters today so I may see the new 7, but not holding my breath...