Hermione is not only smart, powerful, and generally kick-ass, she also is one of the most compassionate characters in the series, which is saying a lot. She's empathetic and loyal, and never hesitates to bust some heads for her friends. Seriously, Harry and Ron would be LOOOONG since dead if it weren't for her. (Or worse, expelled.)
...Also, she slapped Draco Malfoy in the face so hard his head was probably spinning for the rest of the school year because he was being a bully. Need I say more?