Yeah MX Player plays pretty much anything you can throw at it. I also really like how if you slide up/down along the left edge of the screen it controls brightness and sliding up/down on the right side controls volume.
There was a time when the Samsung stock player was top of the line - wide codec and container support and surround sound mode. But the video player apps have gotten so sophisticated it looks like Samsung gave up. The video player on the S2 is better than the one on the Note.