Well, out of my two mains, Moira's the one I actually consistently like, whereas Sio annoys me a lot!
On a more relevant level, I like playing an overpowered space wizard. Then again, when it comes to Sio there's no mystical element, she mainly uses the Force to turn into a force of nature and crush opposition.
NFUs are fun as well, especially since there's more of a challenge involved writing them. I'd be happy if there was an NFU major faction other than the Mandos that can hold its own against the forcer-dominated ones. Of course, my one NFU main, Moira, is not exactly 'normal'.