Anzu = hot childhood friend with delicious absolute territory
Mai = sexy older woman with cleavage
Shizuka = sickly imouto with bandages
Isis = older sister priestess who's tanned
Rebecca = genuis American loli with glasses who throws herself at the main character
Vivian = crazy fangirl martial artist with epic legs
Not to mention all the female Duel Monsters like Black Magician Girl, Injection Fairy Lily, etc. Hell, even Kaiba's Blue Eyes is a hot chick.
YGO! would make the best harem anime ever if it wasn't all about card games.