Name one..... uhm. THAT is difficult. There's so many it's hard to make one, definite choice.
In a sense, it is difficult not to name Dragon Age or Mass Effect 2.. they are movie - LIKE but so much more. Star Wars, in its various incarnations, has definitely helped shape my interests and been a lifelong fascination, but they are hardly the best movies. Many of the recent blockbusters has been really spectacular, but often easily forgotten, as well. I am Legend stands out a bit. As does Avatar, Matrix, Watchmen, and countless others. The Lord of The Rings were great, but mostly, for me, because I've read the books so many times. Peter Jackson's imagery was not so far from what I'd imagined, whcih helped - that is a dangerous risk with having read the books first.
And then there's the old favourites - Dune (Dino De Laurentis), Blade Runner, A bird flee over the cuckoo's nest, the Shining, the Godfather, all the classics that at least deserve a mention - films that retain their potency today (with a few exceptions).
Strangely enough, the film I've seen the most times will have to be it - surprisingly enough, pehaps:
Snatch - Stealing stones and breaking bones.
I've long thought Guy Ritchie makes brilliant films - their attention to dialogue - and monologue - is brilliant. They are perhaps a bit similar to each other in tone, if not in plot, and has some similar characters, but other than that, they are great little films. I've liked all of them, and the reason I single out that one, is that for a long time that's the only one of his films I had on DVD so I watched it a lot of times.
Tomorrow my choice might well be different, though.