My favourite media player has always been VLC but I have to say, FF Player gives it a run for it's money. It plays all kinds of audio and video files quickly and with stability. It isn't packed with advanced features but as long as you've got DirectX codecs installed, it plays a remarkably wide range of formats.
To use it, you simply drag and drop the files you want to play into the playlist. Both the playlist and the video screen can be dragged to the size you want with an attractive slider. The overall feel of the player is very sleek although unfortunately, there are no extra skins that you can add to it.
A super simple little player that can handle almost anything and won't let you down.