+ Defeated Grindelwald, the 2nd most dangerous Dark Wizard of all time. Had him imprisoned.
Worried Voldemort, but never battled him directly.
++ Defeated a friggin' Balrog in hand to hand combat, and then defeated Saruman, the 2nd most dangerous Evil Wizard, who was then killed.
Worried Sauron, but never battled him directly.
++ Brutally maimed Vader, a former friend and 2nd most powerful Sith Lord. Also killed countless Imperial troopers.
Did the Evil Emperor even know about him?
- No known sword skills + Does a lot of sword fighting as well as using staff. ++ Most powerful sword of the three. - Stayed dead, no advice from the grave ++ Came back from the dead as Gandalf the White + Stayed dead, but could offer advice from the grave. ++ Can teleport(!) himself with magic. -- Rides a horse, for days and days and days.... + Flies in space ships, can probably levitate himself with great effort. ++ Faster at magic and does it frequently. - Needs a staff to do magic - Almost no magic, only limited telekinesis.
Imagining all three of them in a fair fight where they all had some prep time, I give Dumbledore the win due to speed, more magic, creativity, ability to control time, advanced planning and deviousness. Of course, Obi-Wan could do a lot of damage with blasters from a space ship... If there was no foreknowledge and you just stuck them all in a room and told them to kill or be killed, ready GO! Would Gandalf's staff survive a direct hit with a light saber? I think not. Obi-Wan is fast, but with a small flick of his wand, Dumbledore would immobilize the other two. I may be missing some important deciding factors, however.
Cast your vote. Write in candidates allowed.