Head - It gets a lot of shit, but I think it's severely under-rated. Besides, Head paved the way for BBS and movies like Five Easy Pieces, The Last Picture Show, and the best road movie of all time and arguably one of the most important American movies ever made, Easy Rider.
Fear and Loathing in Las Vegas - The movie, as always, isn't nearly as good as the book, but it's surprising how good of an adaptation it is. Better yet, just go read the book.
Return of the Living Dead - People are obsessed with zombies these days, but I think it's ridiculous because I've yet to see a zombie movie that compares to RotLD. Everything that's come out since is just pointless.