It was Grindelwald. =p

I agree, it was Grindelwald. Though, seeing as he's not there, I picked Snape. I think sharing a common secret like that creates quite a strong bond... But Dumbledore didn't really seem the kind to have 'best friends'.

Though I actually think that, like a lot of brilliant people, he didn't really have a best friend.
Sad...

I should have made it question clearer. Yes, Grindelwald and him were best friends when they teens, I met later on in life.

WHERE THE HELL IS GRINDELWALD???

snape of course!

It was grindewald but hagrid would be second

this is a question that is very open to debate.
I think that dumbledore was probably closest to Grindelwald however i don't think he would call him his best friend. Dumbledore had an infatuation over grindelwald in the same way that Bellatrix had an infatuation over Voldermort.

Dumbledore preferred to do things alone. He didnt really have any "best" friends. Grindlewald was his young friend, not exactly best friends. But I would say Harry, because he tells Harry everything and says "I'm safe with you Harry." in Half Blood Prince (the cave/horcrux chapter. After I would say McGonagall because he trusted her to make sure things were in order and to make sure Harry was safe. Then it would be Snape, even though they werent really "best friends" or even "friends". Snape was more of a spy for Dumbledore. Although Dumbledore always trusted Snape because of the whole Lily Evans thing. Overall I think his best friend was Voldemort :D!

I was between Snape and Hagrid, but the relationship with Snape was closer and they had a secret very important... sO I picked him

Dumbledore was extremely close to all of these people, but he was like the like person Hagrid had till Harry came along