Reason #6

And finally:
Gandalf defeated a Balrog, he fought the Balrog for 2 days and 2 nights on the peak of Zirak-Zigil
before casting him down to his death until his body destroyed the mountainside. I don't care what you say ,nothing in the HP world begins to compare with a BALROG. Plus: A Balrog is a corrupted Maiar, and it takes huge amount of power to destroy one.

So in the end GANDALF WINS!!!!!!!

Reason #5

Gandalf and Elrond summoned a huge flood at the ford of Bruinen which nearly destroyed all of the landscape and swept away all of the 9 nazguls. WHAT YOU GOT DUMBLEDORE?

Reason #4

While Aragorn and the hobbits were making their way to Rivendell Gandalf single-handedly defeated all 9 of the nazguls on weathertop by summoning fire and lightning  for a FULL NIGHT!. Okay, I'll give Dumbledore this: he can probably summon fire and lightning, but keeping it up all night???? that's just WTF?!

Reason #3

In The Lord of the Rings ,before the fellowship came to Moria Gandalf grew to giant size and beat the shit out of the orcs and wargs with 2 giant tree branches that he himself set on fire. I NEVER SEE DUMBLEDORE GREW TO GIANT SIZE!!!

Reason #2

During the timeline of The Hobbit Gandalf and the white council put forth their power and drove Sauron (a.k.a "The Necromancer" as mentioned in the book) out of Dol Guldur,Mirkwood. You gotta be packin some magical heat to drive out the dark lord himself, and NO, the dark lord is not Voldemort; Sauron was the dark lord 50 years before Voldemort was even a twinkle in J.K Rowling's eye

.Gandalf vs The Balrog, Khazad-Dum

Reason #1

Gandalf is IMMORTAL, he roamed Middle-earth for 2000 years before Sauron was finally overthrown.
He's not a man, he's ahigher being known as the Maiar who is second to the Valar under the high god Eru Illuvatar; Dumbledore is just some dude.