Ouran High School 9/10 - very catchy!
Haruhi OP 8/10 - Catchy and slower paced than the fast ED
Bleach ED - 7/10 - flows along well with the animation
Pitaten OP - While not as good as the ED song they did for Popotan, it's still catchy.
Nadesico OP - A classic
Paradise Kiss OP - Stylish and very catchy.
Honey and Clover OP - Possibly the most creative (and tasty) Op ever.