If I was going with the surprise factor, I guess I'd like to see Psycho again for the first time. But not as I actually saw it for the first time, which was with full knowledge of all the twists in the film. I was unfortunately born into a time when Hithcock's classic and both its shower scene and its ending is completely embedded into pop culture and the collective conscious. I'd prefer to go back to 1960 and see it as everyone else saw it for the first time, when it somewhat revolutionized the moviegoing experience and spawned the very concept of movie twists and spoilers.
Then there are the movies I loved as a child and have nostalgic appreciation for. I wonder if I saw them for the first time (as more of a film cynic and snob) now if I'd still like them as much as I did then. Examples in mind include The Goonies, Explorers, Back to the Future, Return of the Jedi, Popeye, Superman (and definitely parts II and III), any Muppet movie (without having seen any of them nor the Muppet Show or anything else Henson-related), Disney's Robin Hood, Ghostbusters, Ferris Bueller's Day Off ... and so on.
I guess the one movie that I really would like to see for the first time in order to experience it completely new again is Heathers. Thanks to Monika's inaugural Cinematical Movie Club viewing the other night, I wondered if anybody was seeing the movie for the first time. Given that I grew up memorizing every bit of it -- both from watching the movie and reading a copy of the original script -- I do wonder what I thought the first time I heard some of those brilliant lines and saw some of those twisted deeds. Oh, I guess Duck Soup would be another. Will Goss' recent Basics column made me jealous of him for being able to experience the Marx brothers for the first time.
Honestly I think I'd rather know what it's like to go back and re-read some of my favorite authors for the first time. Most of my favorite films, on the other hand, are those that improve with each viewing and have become ingrained in my brain and memories. But because I especially love the script to Heathers, I think I have an attachment to it that's similar to that of my favorite books.
As I said, it's a difficult question to answer with only one film. Can you do it?