My argument for Dr. Strange has to do with his area of responsibility.  He patrols the multiverse, which used to just be called our dimension and alternate dimensions.  Other superheroes like Spider Man (who fights with Dr. Strange for my top hero spot), are really local; “Your friendly neighborhood Spider Man”.  Many of the big comic heroes concern themselves with all of humanity, or planetary safety.  In my opinion, Earth threats are small potatoes compared to dimensional aggression!

There have been so many versions of Arthurian legends, but in all of them Merlin is super powerful, although you don’t always know exactly what he can do.  The British t.v. show Merlin gave him telekinesis along with a lot of ritual spell casting for example.  I think Merlin would stand up well to Gandalf or Dumbledore, but Strange would put him down!  Also, he is mainly responsible for Arthur, Camelot, and maybe England; not even all of Earth or Middle Earth, like Gandalf.

Returning to the currently more well-known Harry Potter Universe, you could say that Voldemort, Hermione, Harry’s Dad, Snape, or Sirius were more powerful than Dumbledore and I probably wouldn’t argue (well, I would argue out of habit).  However, only Voldemort tried to affect the whole world and he didn’t succeed.  Which brings me back to Dr. Stephen Strange, who is the best pick for top wizard in the genre!