While its not a series (its a movie), Forbidden Planet tops my list.
Another very cool movie is Fantastic Planet.
AFA Star Trek goes; its a toss up for me. The original Star Trek is always a favorite - it's where I cut my teeth. TNG is better in many respects, but I never did like #1. IIRC, I did like all the Enterprise characters in the original ST. My favorite ST and TNG characters are Kirk, Spock, Picard, and Data (not necessarily in that order). I was disappointed that Lieutenant Yar's character was short lived. My favorite recurring character was
Q. John de Lancie did a fantastic job with that character.
I don't subscribe to cable TV, so I guess that's why I have never heard of Firefly.
