Who are the most powerful Star Wars characters canon AND Legends and why?

Count Dooku is one of the most powerful in body and force but not in mind as he was used as a pawn for Palpatine and later discarded.

I'd say either Luke, Palpatine, or Darth Vader. If thrawn had competent underlings, he would be unstoppable.
Additionally, some other reasonably powerful characters Yoda and Mando.