While not movies, I've gotten some enjoyment from watching Youtuber's reactions to watching various episodes of Doctor Who (particularly the 9th, 10th, and 11th doctors) and Buffy the Vampire Slayer, for the first time. Seeing their reactions, reminds me somewhat of mine, so it's a bit like watching for the first time. Finding out who River Song was....mind blowing, so neat to see people reacting to that, and so much more stuff.
Of course there are people who react to movies, too. But it was reactors to those two TV shows that really got me started to see those.
Ironically, perhaps the biggest surprise in movie history, or at least sci-fi (I'll give the 6th sense some props), was totally ruined for me, a guy to wanted to impress us with his "inside knowledge" in 1980, told us who Luke's father was....weeks before The Empire Strikes Back opened. So, I never got to live that moment as a surprise when I saw the movie. Still P.O.'ed that he did that. I have seen Youtubers react to that scene though, often with their kids.
And oh yeah, the 6th Sense got spoiled for me too. I did not go to many movies, watched most on pay cable like HBO/Showtime, and someone gave that away before I saw it.