I use AVPlayer. It's ok only sometimes I have a problem to set it up so that it correctly displays subtitles in my language, but otherwise it's good.
I've also used OPlayer previously, but there seems to be a problem with the latest update, it seems every few minutes the video/audio stops for a split second which makes a very unpleasant interruption if you watch a movie. But it could be my iPod, I dunno...