Evolution
I second “Showgirls” and add “Glitter” and “Wisegirls”, especially the latter. Great/campy mob film. (OK I just like to look at Mariah, SUE ME!) 
I seem to remember John Carpenter’s The Thing being critically loathed when it was first released, as was Big Trouble in Little China. I loved both movies and have seen them probably a dozen times.
Actually, I don’t know how John Carpenter keeps getting his movies funded; they almost never make money, and almost always get terrible reviews.
I loved Pitch Black well before I knew who the heck Vin Diesel was. I thought it was extraordinarily innovative in the way it played with alien and space movie conventions. It was given almost universally mediocre reviews.
I loved AI. Think it is a masterpiece. The last 20 minutes were dreadful, but I thought Spielberg did a wonderful job with most of the film. I am actually glad Kubrick never made it. Kubrick was a great conceptualizer of films, but the end result always left me cold. I never felt he had any kind of touch with his actors.
Ghost Ship. I love scary movies that are about ghosts-rather than your typical slasher films. I love a good ghost story and I love mysteries. And Monica Mancini’s singing was gorgeous!
tracer, I’ll second that vote for Shallow Hall, and I’ll throw Mallrats out there. Yeah, I know, it was pretty bad, but I thought it was entertaining.
‘A life less ordinary’ and ‘Frequency’ are excellent choicees! Whole heartedly agree.
And may i add a few more: ‘Anaconda’ and ‘money train’ were both great entertainment.
And is it just me, or does every movie thread here come dangerously close to a debate on the merits or demetirs of ‘A.I’? Just like the way every thread at aintitcool becomes a starwars or lord of the rings free-for all…
For an extremely related recent thread, click HERE
Barry
Two already mentioned here: Lynch’s Dune (far superior, I thought, to the dull thing they put on cable not long ago), and AI.
Red Planet. I was at least glad to see that The Bad Astronomer liked it too.
A movie I liked that got mediocre reviews was Wildthings. For a movie that looked like just a skin flick it was remarkably well done. All the plot twists really kept you guessing. The movie really surprised me.
Fifth Element
I second this. I loved Death to Smoochy, and couldn’t understand why nobody else seemed to.
I have to completely agree with AI. I thought it was brilliant. I can even take the last bit even though I know it’s inferior to the rest. It should have ended underwater, but I’m okay with the fact that it didn’t.
I have to respectfully disagree with Shallow Hal. It deserved every rotten tomato it got. A movie based on the idea that inner beauty is, inherently, thin? I’ll pass.
Thirded. Edward Norton’s a god and Robin Williams was great as the guy that’ll do anything to stay on top. The Nazi Rally setup was hysterical and I think I laughed for five minutes straight when at the end, Rudolph answered “I’m fucked up in general so it’s hard to gauge” when asked if he was okay.
Oh, and I didn’t hate Star Trek: The Final Frontier. It was so bad it was good.
I also like “The Postman.” In fact, I’ve been thinking of reading the book. Has anyone read it?
An older one is “Paint Your Wagon.” My whole family loved that movie despite it’s being on every shitty-movie list since it was made.
I really dug The Postman, in spite of Kevin Costner’s wooden acting. And, hey, how can you not love a movie that has Tom Petty in it as, well, Tom Petty.
I also liked Battlefield Earth. Great, cheesy fun, and John Travolta was hilarious as an alien. 'Course, I’ve always had an appetite for badfilm that actually rivals my appetite for good horror movies.
A.I. was horribly misunderstood by many. I don’t even have a problem with the third act, since I feel it fits in rather well.
Can’t explain why I like The Fifth Element so much.
Gladiator has taken a surprisingly large number of lumps, but I think it’s a great flick.
Raising Arizona was given an abysmal review by Roger Ebert. I think it’s a brilliant flick, and until The Big Lebowski came along, it was my favorite comedy film.
Some have listed my favourite bad-reviewed films too - Dune,
The Fifth Element - but I’ll put in a vote for Cutthroat Island (guess it’s the piratey influence from Monkey Island) and also **Dead Man on Campus **(can’t explain why I like it though!)
i really enjoyed Blair witch 2, I found it quite frightening, unlike evryone I know, who says its bad.
I also really enjoyed The Faculty too.