VLC allows for all kinds of post processing. Like you can sharpen, use different deinterlacing methods, change color/contrast/hue/brightness. It has a bit more variablity in the rendering and has some software elements to it.
All other players that don't really have any options like MplayerX, Quicktime, Movist rank pretty evenly on the battery life scale.
I am guessing quicktime would be the most efficient if there is any difference. But if FFmpeg is running its makes no difference which of the other players is really used.
VLC just offers the best viewing experience. If you apply a sharpen filter on 720p content you can get it close to 1080p subjective quality. Or at least half way there. The filter needs quite a bit of cpu power though. Everything has a cost. Audio can also be adjusted if needed.
VLC is still the best overall I think. The others I would only use for long on battery watching and there I would still prefer MplayerX over Movist.