Kuwabara is an excellent character. He is an entertaining goofball who provides Yu Yu Hakusho with outstanding comical relief. Not only that, but he is an honorable and loyal young man with a heart of gold. He would make a great friend.
The majority of Yu Yu Hakusho fans dislike Kuwabara because of his physical appearance. Truth be told, he is not a good looking guy (in fact, he is quite unattractive). However, that is not a good enough reason to dislike him. Kuwabara may not be attractive, but that does not make him a bad person by default. He is a cool guy.
Fangirls LOVE Hiei because he is a bad-*** with a cold, stand-offish demeanor. Personally, I have never been fond of his character. He is your standard angsty jerk. As far as his "hawtness" goes, I have never found him attractive. He is a vertically challenged twerp with odd spiked hair, a sour face, and a third eye.