iPads already contain QuickTime. They can play native mp4 and m4v files: just transfer the movies to your iPad's Videos app using iTunes.
If you're looking to play other file types, such as flv, wmv, or avi, I'd recommend OPlayerHD. It's well-designed, and so far it has played everything I've thrown at it. There's a free version too, if you want to test it out.