FRED and GEORGE WEASLEY hands down. Totally mental, incorrigibly hilarious and so not square! They alone can say, "Honestly woman, call yourself our mother?" and get away with it! But the fact that their characters aren't afraid of anyone except their mother, I find kinda sweet.
They break rules but had set their own limit ("we've never been expelled, have we?"). They don't seem to be bothered by their family's lack of money unlike Percy and especially Ron. I love their view of life as depicted by their carefree and happy-go-lucky attitude and antics. They alone provided me with the comic relief everytime I come across them in a scene. They could turn even the most serious and dangerous moment into a laughing matter. Who could ever forget their beards when thrown out by the Goblet of Fire? And their great escape from Umbridge?
We all felt that with the darkness and the evil, the twins somehow seemed untouchable, that because they were funny, evil can't touch them. But, alas, even their lightheartness wasn't shield enough from the evils of Voldemort.
And I think that with all the deaths in the series, none had the most impact on us than Fred's. We were all shocked. We knew that there would be a lot of casualties in the process, Harry's death itself was almost expected. But that a Weasley twin actually fell dead? When I read "No, Fred! No!!!", I was not able to turn the next page to read more. I had to go back and reread that parts thinking I missed something. Half-thinking I made a mistake in reading. But darn Jo Rowling, she had to take Fred!