"greatest" is relative. It all depends on skill and power again. I would say BodySong, MindSong and SoulSong be the most powerful and hardest. BodySong controls animate objects, MindSong controls inanimate objects and grants telepathy, and SoulSong allows you to speak with dead souls as long as you can find them, you can also coax them back into their shells (basically necromancy), to control it once it's back in it's shell is to use either BodySong or MindSong as the corpse is half-dead, half-alive. To not control it is to unleash an angry and hateful zombie/draugr as the soul hates tasting life a second time, it will endeavor to destroy it's caller if the Weaver doesn't take control the moment the soul enters the shell, then will commit suicide, and taste death a second time.
As for the hardest to tune to....FateSong, SoulSong, DecaySong are the ones I can think would be hard to tune to right now. Things might change.
As for what does the most? Those would be the elemental songs. The are by far the easiest and have the most raw power, plus with skill, time and patience one can learn to do all sorts of things, and combine Songs to do new marvelous things!
Aww thanks dear