Albus Dumbledore
Sometimes he wondered if he made the right choices. There were so many times when he thought what he was doing was right, but he had never imagined the consequences of those actions.
Did he make the right choice inviting Tom Riddle to come to Hogwarts? Of course, at the time, he had no idea that the young orphan boy would start two wars, but still. It all could have been avoided.
If Tom Riddle hadn't known about his powers, he wouldn't have been able to rise to power like he did. Hagrid would never have been expelled, and Moaning Myrtle, and many others, would still be alive. Harry and Neville could have grown up with their parents, the Weasley children wouldn't have been put in as much danger, and people would generally just be happier.
Then, there was the matter of letting Remus Lupin into Hogwarts. Albus knew that that was the right decision; the boy deserved to come to Hogwarts more than anybody. So what if he turned into a wolf once a month? How did that take away his right to have a good life? The reason he wondered about it was because of how much Remus had lost from it. He made great friends while he was at Hogwarts, but he lost all of them in the war. James, Lily, and Peter all dead. And the worst part was that they had all died because of his other friend, who was then in Azkaban. Remus would have to deal with that for the rest of his life. Did he really gain more than he lost from coming to Hogwarts?
Oh, and let's not forget Harry. Albus had been thinking in Harry's best interest when he made the decision to leave the boy with the Dursleys. He thought that it would be good for the boy to have a normal childhood, one without the fame of being 'The Boy Who Lived.' One where he didn't have to know that his godfather was the cause of his parents death. One where magic was nothing but what kids dreamed about. He thought Harry would do better growing up like that.
Albus had hoped the Dursleys would treat Harry well. Instead Harry had lived in the closet under the stairs for the first 11 years of his life, and hardly ever got to eat. Harry grew up thinking there was no one out there who would ever care for him. That was not what Albus Dumbledore had wanted when he left James and Lily's baby on that doorstep.
Now, he would always wonder how things could have been different if he didn't make those small mistakes- how many lives he could have saved, how many people would have been better off.
He would always wish he could change what he did.
Because everyone makes mistakes. Everyone regrets something. And everyone wishes that they could go back in time to change something.
Everyone.
Even Albus Percival Wulfric Brian Dumbledore.