All of the girls have epic sex potential.
Moka has her innocent side, easily taken by Tsukune, and then her true form, who can easily take Tsukune. Sex switching back between the two of them would be epic and use up most of your energy.
Kurumu has the boobs and the potential for mind slave control. There's a lot that can be done with that.
Both Yukari and Rubi have unlimited magic potential, and magic and sex go together quite well.
Mizore...oh, Mizore. Assuming that she gets Tsukune worked up enough so that the cold doesn't affect his manhood, I can see slow caresses and a lot of use of the tongue. She's good with that lollipop.
And maybe Nekonome-sensei. A cat is fine, too.