What's your all-time favorite Sci-Fi series/franchise? Why?

Movies, TV series, book series, pick anything. What's your all-time favorite? What universe do you love to get lost in the most?

Star Wars is definitely up there for me, and I'm also a Star Trek fan, but I think the ColdFire Trilogy by C.S. Friedman has to be my all-time favorite. Such great characters. If you haven't read trilogy, definitely check them out, you won't regret it.