Didn't someone say Gandalf was only a 5th-level Wizard by D&D standards? (Except maybe the whole coming-back-from-the-dead thing.)
The problem with comparing magic-users is that each author's concept of magic varies so wildly that trivial powers in D&D are rare in low-magic worlds, while in others ordinary mortals can sometimes wield what would be world-shaking powers in D&D.
Describe Paul Atreides in D&D Psionics terms. I dare you.
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)