I've always liked and, for most playback purposes, recommended ProPlayer (AppStore link). While it isn't exactly feature-packed (you won't find, for example, audio, saturation or brightness boosting in the app), it has (generally) a well-behaving and -performing set of video and audio decoders. Most of these decoders are exactly the same as in AVPlayerHD, one of the best iOS multimedia players.
The latest player update description boasts “Hardware acceleration” and states the new version is "a major feature update":