My absolute favorite involved the Share (by Fun Factory). With the short end inside of myself, my girlfriend gives the other end a blow job. The visual is incredible and the motion of her mouth and hands moves the share just enough to stimulate my g-spot. It gives me the wonderful feeling of being dominant and as soon as I orgasm, she's able to hop right up and ride me. (=
It really depends greatly on my mood. Oral sex is a favorite, especially with fingers inside. Fisting is incredibly intimate and amazing. Strap on sex is brilliant. I don't know if there is any one single most gratifying act.