Absolutely Pacific Rim. I don't hate Avatar...I just feel it doesnt warrant more than one viewing.... Pacific Rim - it's just a better all-round movie...it's a dudes' beer n pizza movie, in a Starship Troopers-esque way.
Avatar is a better movie from a technical aspect. But rim is much more enjoyable. And has much better replay value.
Watch them both only ONCE.....and I been having the argue to watch 'em both again....Pacific Rim more so than Avatar...Avatar is too much lovey lovey shit...boy meets girl.....girl hates boy....boy doesnt' care for girl...but then they spend a few days together...boy loves girl...girl start falling for boy but family(her people/king) wouldn't approve...boy or girl risk everythying for the other one.....girl feel betrayal.....etc....bullshit.
I also saw both only once and have no interest in seeing either again. If I was forced to rewatch one of them it would easily be Pacific Rim as I find the film much more interesting and original. Rather than rewatching Avatar one should instead rewatch Dances with Wolves, a much much better version of the same basic story.
Upon first viewing I was greatly disappointed by Pacific Rim. Like it better upon second (and last) viewing. Avatar I watched it twice at the movies, bought the blue-ray of the expanded version and watched it a few more times.