8 XXX : transformation

Cows and hucows, definitely.
It's hard to go wrong with a succubus (they're literally made for sex), but I tend to prefer the execution to the concept. There are some nice stuff with cow- or slimegirls too, though.
I ain't answerin until I see myself represented I'm kidding. It's kitsune
Probably Succubi or Hellhounds.
Mermaids!
I tend to like a lot but mostly slime, because the way the person's "texture" change and how they react to that is always super interesting. I also really like insect monster girls/boys because the foreigness in comparison to the original human state also creates very interesting reactions. (I also love cow and mermaids but that's mostly because I already love cow girls and mermaids to begin with)
Lamia's and Cecaelia
Dragons, of course!