Mine's the Cube version, too:
Best:
Voldo
Maxi
Nightmare
Voldo and Maxi are both button mashers, so they're easy to use, and played a lot of the SC2 demo with Nightmare, so I'm highly skilled with him.
Worst:
Ivy
She has soom of the coolest, but hardest to pull off, moves in the game.
The Mighty Sharpedo says "Insert proper sig here".