Another one with too many categories. I kinda covered my favorite humanoid back at Day 2, and I suppose "Natural" was covered a few days ago when I mentioned my like for the badger (though, admittedly, "Natural" is an oddly named category). This, of course, leaves me only with the Fey.
I'm a big fan of the fey. I don't think they get enough play in most fantasy rpgs, as historically, they made up most of our folklore of ancient cultures. Even most of our common fantasy monsters and races were once fey - the goblin, hobgoblin, gnome, elves, dark elves, kobolds, dwarves, and trolls, at one point or another were vaguely categorized as fey. This isn't to mention creatures more closely related to what rpgs have most commonly attached the label to, like sprites, naiads, and satyrs.
My favorite, however, are pixies. In my Wilderlands campaign, they are usually heralds of a larger fey presence or greater fey power, and are passive-aggressively mischevious, often trying to provoke those they torment in front of their unknowable masters in hopes that they will see the ancients use their powers in the real world.
That game aside, a pixie's natural invisibility, along with their small size, makes them ideal scouts and spies. A stereotypical pixie is also a fun, playful companion who is always curious and ready for adventure. In another campaign I ran many years ago, my friend, Kortney, went another direction with a pixie fighter who came from a very militaristic society that resembled the lifestyle of aggressive ants or wasps.
And again, a creature type that is widely represented with images on the net, but with not many that convey the traditional rpg pixie.