Who are your favorite characters from any sci-fi/fantasy TV show or movie? Please indicate what show and why they're great.

I'll start with just a couple of mine:

Scorpius (Farscape): one of the best bad guys ever, would elaborate but oh so many of his best moments revolve around big spoilers.

Rodney McKay (Stargate Atlantis): best combination of arrogance, lovability and heroism ever on TV