My fav idols off the top of my head were probably Mai Hoshikawa, Alice Otsu, Marina Hiiragi and Rino Yuki. Yayoi Mizuki is good too but i'm usually dissapointed when she cowgirls becuase she usually stops early. Also if Nao Jinguuji used her ass more she would be in my top 5
EDIT: forgot Aika...