Bold of you to assume I settle on one.
But here are my favorites:
Rodenian/Mouse
Bat
Leithan
Deertaur (For a sense of symmetry to dear Amber.)
Synth (This one requires the save editor.)
Behemoth
Sex is also very much in flux. Ideally, I would like to be able to change from having a dick to a pussy quickly and easily, but often the simplest solution is just to have both.
The game is full of transformatives, and for that reason, I find it very hard to settle on one thing. Thank goodness for the nano-surgeons or I would end up just as tainted as Daddy Steele.