They belonged to a poor family and still had the guts to defy convention and utilize their passion for fun in starting up. They were the height of cool at Hogwarts and some of the stuff they did was supposedly exceptional magic, although they never got the best marks. They went to school because they loved it, and when they did not, they calmly flew out. If you notice, when they flew out, massive iron balls were attached to their brooms. I don't know if this was deliberate, but yes, they had balls. Yet, they had their hearts in the right place. They were among the first to sign up for the D.A. in their final year. They fought bravely against the dark side and suffered hardships with a laugh.
Rockstars, the pair of them; rebels who followed their passion. Fred and George Weasley exemplify what nearly all students, particularly those in college, want to be.
P.S. Given a choice, I'd call it a tie between them and Mama Weasley. A woman who can cook, wash dishes, fold clothes, knit jumpers and, in her free time, kill the most powerful Death Eater to protect her kids, is clearly to be reckoned with. Read Stephanie Vardavas's answer for a demonstration of her awesomeness.