My favorite are robot, ghost, and angel in that order.
Curious robot boys that desire to be like humans make me feel warm inside.
I like ghost boys for the tragedy they tend to bring to stories and the desire to see them move on and be happy
And Angels are just pure and adorable.
Vampire, Monster, and Demon could be good too, but only if it's an evil shota, or one that takes pleasure in doing evil things, like Laharl from Disgaea.