My guess is MP3. MPEG I was introduced years ago (around 1992?), and it was designed to run on all kinds of processors, even the slow ones (like in the cheap mp3-players nowadays). WMA is a lot newer, and using less bandwidth to achieve the same quality usually means more consumption of processor power. And Vorbis... dunno, I don't think it's really optimized to the bone
. I think it's about as heavy as WMA.
On the other hand, it's not like I notice the difference in playing different formats. Modern processors don't care about 3% or 4% processor load, and my mp3-player can play them all at maximum bitrate.