While I don't consider OPlayer (HD) (AppStore links: iPhone; iPad) the best-performing or most versatile iOS video players out there, it still has some very strong and well-supported features like a very good MSMPEG4 and OGG Vorbis video decoder.
The just-released, new version boasts, among other things, support for volume boosting - something I really need when in a noisy environment or when needing as large a video playback volume as possible (see the annotated part of the screenshot; click it for a larger version):