Well that's a tricky one...oh wait...it isn't. Melkor!!!
This is my order: Sauron > Gandalf > Dumbledore > Voldemort. Sorry Voldie...you lost.
I disagree about King Arthur beating Aragorn. And how does it affect that Arthur is an "original" king and Aragorn "only" long descendent of Isildur? Remember that Aragorn was able to call some back up...the dead, remember
This is an interesting thread. How about Quickbeam vs Treebeard?