In terms of playing opposite of, I'm quite fond of any kind of sea creatures (mermaids especially, but also quite like serpents) mainly cause I enjoy the ocean, swimming, and the breath play elements that can come of it. Beyond that, drivers/half-arachnids, cause I'm personally terrified of spiders but the idea of a half-lady one has some.. odd carmal appeal, plus, bondage and webbing. Lamia as well cause, for whatever reason, I like scaley things and the idea of getting coiled up for wtever purposes. Bonus points if the lamia is a Medusa as well, even.
Playing as, mermaids are number one. What can I say? >.> I had some fun playing an Alraune once, as well as an djinn, and dullahan. But I don't be one too often, not even my favourite, to have others I can say I definitively like. I do prefer playing outside the more common kinds for sure though.
I think what I find appealing is partially the fantasy aspect of it, and just... in general liking monstrous creatures. The inhuman appeal, I guess one could call it. I do like some full on monsters (female or otherwise) as well, but no favourites there I could particularly point to.