Avatar is among my favourites. Would rather call if a favourite thing than a favourite film, but it's still up there. I've seen it almost as much as The Matrix and Star Trek from 2009 - if it wasn't so difficult to sit through, I would have seen it more.
It's the only thing ever to actually depict a complete alien biosphere, and do so in startling detail. And the only way I know of to make you really feel you're on a different inhabited planet. It's the closest thing we have to that, for better or for worse. I hope James Cameron does something good with the sequels, especially crank up the storyline at least a little.
The world it shows is a little too similar to Earth, but what it loses in similarity it makes up in going a little bit far in the fantasy department. Maybe too far, it's almost a fairy tale fulfilling childhood fantasies like floating islands, but shockingly, even that remains mostly within the limits of the laws of physics. Which is also amazing, because what you see on the screen is unquestionably impossible, yet a serious reflection afterwards makes you realise it technically could happen, more or less. That still leaves it a great visual extrapolation of what exciting thing may actually lie somewhere in the universe – it's probably far far from any reality, perhaps being too shy and too bold at the same time, but it's a step in the right direction in the way to imagine things, and to show them. For that reason, I hope it has impact.
It's the polar opposite of Star Trek, where all the magic in the universe has been summoned and turned into nonsensical technobable, but that is seldom used to actually take you to a place that's as exciting... even storywise, because obviously the budget wouldn't allow visual trips to anywhere other than dressed up Earth. Many of the Star Trek stories did use the magic to its full potential and still blow my mind, but that tends to happen when you have almost a thousand episodes.