The movie I most closely associate with D&D is a movie that I've never actually seen: 1981's Dragonslayer.
The reasons for my association are obvious: it came out in the summer of '81, shortly after my 12th birthday, when I was 6 months into my D&D career. In the thralls of early D&D obsession, I was avidly reading anything that even vaguely resembled "fantasy." Then along came a movie--a real live movie!--about D&D stuff; it was confirmation that what I was going through was a legitimate cultural phenomenon. Unlike all the obscure books that I was reading that otherwise went unnoticed in the nether regions of the local library's science fiction section, everyone who saw the ads on TV or the posters at the local cineplex was aware that here was a movie about dragons going on: D&D in film format! And even if no one saw it--it did not do very well at the theaters--it still put the fantasy genre on the cultural map along with James Bond and Indiana Jones--"For Your Eyes Only" and "Raiders of the Lost Ark" being two movies I did see that summer. And even though I still haven't seen the movie, I did read the novelization and the Marvel comic book version.