Recently I saw Chasing Amy for the first time. I was expecting a run of the mill romantic comedy, and then it unexpectedly turned into this heavy and kinda sad film.

Another example is Heat, which I also recently saw for the first time. I was expecting a Face Off / Broken Arrow type quality action film, only for the film to quickly become too awesome for words!

Mad Max is the most recent example for me. Michael Bay had me thinking that I was bored with the action genre, but then George Miller came along and blew anything Bay has done in recent memory completely out of the water.

Fight Club
Fury Road
Let the Right One In
Dawn of the Dead (remake)

London has Fallen, just last week. With the scathing reviews and the obvious "sequels usually suck" attitude, I was not expecting too much. And as it turned out I had a blast. Now, it's not a great movie. It's stupid as hell, but it's so much fun. It's the old-school "are you a bad enough dude to rescue the president" (literally) kind of thing I have a soft spot for. I enjoyed all the bad dialogue, it's all as it should be. And it has some legitimately good action. In fact, I think it has one of the better running gunfights this side of Heat. And Gerard Butler gives it his damndest, and it really works. He and Aaron Eckhart elevate the movie, and make fun what could be cringeworthy.

If I'm being entirely honest- I liked it more than Deadpool. It has better action scenes, and just feels more fun because it feels to me like it nails what it's going for effortlessly, while Deadpool often tries way, way to hard.

Others that come to mind:

the original Fright Night- I expected it to be a stupid teen comedy, but it turned out to be a very good vampire movie, a great mix of horror and comedy. It most surprised me how well it gets the creepy, sexual predator vibe of the vampire.

Ant-Man- nothing great, but at that point I was just surprised that Marvel managed to make a watchable movie, instead of something utterly shit.

Predators- After Avp Requiem and with Adrien Brody as a bad-ass commando, I was expecting it to be a train wreck. Turned out to be a really good, proper entry in the franchise.

Postal was definitely far better than I was expecting from a Uwe Boll film based on those games.

Crank.
We went to the cinema expecting to see a crappy movie.
It was great fun.

Black Coal, Thin Ice

It's about a police officer but it's not what you would expect after all the crap Hollywood produces. Very good.

The Assassin

Another non-Hollywood film that you wouldn't expect from something named like that. I'll give a warning though, there's not much happening in terms of plot or characters' action. Its strength is cinematography.

Amélie

I knew this was a classic and expected it to be good but not that good. There's similar setup called of a movie called
My Afternoons with Margueritte that is also very good. The very thing about these 2 movies that I like is that they show how banal and ordinary yet weird people are. The details are important too.

Fight Club
Ex Machina (The Asian one had nice tits)
Hunger Games Series (I found whole series crap)

Inception. I saw it almost randomly with a family member and his friends. I did not know much about it but I was really surprised at how good it was.

Star Wars Force Awakens. I had heard some troubles about it and questioned it since the film dropped the extended universe (at least the vast majority of it) but it was better than I expected. It does make some references to the extended universe (the identity of the new dark side user is a reference) but I was also disappointed at how it seems to be ditching the war with another galaxy concept and how it felt like it was ripping off from its own franchise in essence.

Pirates of the Caribbean: On Stranger Tides. It was shockingly good IMO. It did not try to rehash the same jokes as the third one did or part of the second one. It also had a new villain and was part of a story arc onto its own. It actually felt fresh despite being the fourth installment.

Inglourious Basterds
Suck
The Devil's Rejects
The Lone Ranger
Haunters (2010 Korean movie)

It's a disservice to go into a movie with expectations either way!

Heat: Excellent movie. Make sure you watch Collateral too!

Fight Club: This film has not held up over time, I used to think it was okay, saw it a few times back near when it came out/a few years after and enjoyed it. I then tried it a few years ago and I couldn't stand it it was so bad, and I love Brad Pitt and Edward Norton, but I was just "no!" and had to turn it off.

Battle Royale (Fuck The Hunger Games movies)

I´m not a fan of Adam Sandler, but Little Nicky was somewhat enjoyable.

Curiously, that's probably the most hated of all his films.

Really ? I thought it´s one of his movies that had a certain charm to it. :D
The one in which he has to take care of his exes son was okay as well. Forgot the movies name though.

And while I´m at it, Starship Troopers was really good.

Zardoz of course.
Rollerball (1975).
Possesion (1981).
Pain & Gain by Michael Bay.
Postal and Rampage 1+2 by Uwe Boll.