Cool Hand Luke is accurately rated. It's considered one of the quintessential badass movies, and that's exactly what it is.
Pan's Labyrinth is extremely overrated. Its fantasy world is disappointingly rendered and its violence and brutality belong in a different kind of movie. The "fairy tale for adults" approach is played out and Guillermo Del Toro didn't add much to it. I prefer both Hellboy films. I would like to see this script executed by Hayao Miyazaki, however. He could have brought a visual beauty to the labyrinth with his artwork that Del Toro failed to bring with photography.
The Sixth Sense is underrated. Sounds weird to say it, but it seems like this movie has been all but written off critically in recent years thanks to M. Night Shyamalan's ugly downward spiral and the then-shocking plot twist has become shorthand for lazy filmmaking. This is one of the best suspense/horror movies of the last twenty-five years, though, and just because The Happening exists doesn't mean we get to make fun of The Sixth Sense.
Avatar is so terrible that offering a relative rating compared to public consensus would be useless. That Best Picture nomination was a goddamn travesty. It gets a lot of credit for its visuals, but, well, it looks like an XBox 360 game. I'll just play one if I wantt that, thanks. The rest of the film is far worse. The script, the plot, the performances, all dreadful. The only high point was that Zoe Saldana is actually so hot that I was slightly turned on by a computer rendering of her as a giant amazonian blue person.