Hey, I'm Vice and I'm doing this so YOU can pick the right fapper for tonight.
Hentai Subgenre: Tentacle Rape.
In short: Dude get turned into tentacle monster that rapes some magical girl. But honestly it was the magical girl's fault that he became the monster and she (the magical tsundere) feels terrible about it. He becomes the monster from time to time to basically have sex and turn back so tsundere lets him take her as the monster by the end of the first episode. He can start to control his powers in the second episode. And a villain appears in the first episode. All of course has plenty sex, but a decent storyline.
Best Scene: In episode 2 he plays the guilt trip on tsundere that he became abnormal until she eventually lets him finger her in the pool, but he wants more and pulls her in for real sex. This scene builds up the actual best scene when discovers he can become invisible... He rapes tsundere, invisible, while she's giving the class a presentation. She tries to keep up the speech and hold back orgasm as invisable fingers, tentacles and dick are individually stuffed into her pussy and play with her breasts... she cums...