Really, really tough call. I think I can narrow it down to eight films, but not much further than that - and then I'll probably think of others to add to the list, but I think I'll go with the Back to the Future trilogy. I really don't want to whittle it down any further than that - they all have something to recommend them, be it the fabulously tight plotting of the first film, the wibbly wobbly timey wimey nature of the second film, or the sheer joie de vivre of the third.
The other five, in no particular order, are the original Star Wars trilogy (although I'm less keen on Empire than I am on the other two); Serenity, just for being awesome and a reminder to George Lucas of how it should be done (it came out several months after Revenge of the Sith - and the Hitch-hiker's Guide to the Galaxy movie for that matter - and blew them both away, IMHO); and 2001: A Space Odyssey for being a seminal, if rather confusing, masterpiece (the book makes far more sense - every time I read it I think, "oh, so that's what it's all about!" and then I go back and watch the film and I end up scratching my head all over again...).