The first gender bending manga I read (as far as I remember) was Soul Chain by Ruen Rouga. It was brilliant, and very very hot for me. It's about a prince exiled from his land who finds a witch in a forest. He swears his entire being to her in exchange for the power to kill the invading king. So the witch basically turned him into her bitch, pretty much breaking the prince through very hot steamy sexual stuff, giving him tits and even a pussy eventually.
The manga impacted me in a huge way. Many of the kinks in it stayed with me till this day. And it has a very beautiful story to boot.