I never got into Lost or 24 (or Law & Order) so they aren't on my list.
My top 5 (I can only pick five?? AUGH!) would have to be:
- Firefly
- Babylon 5
- Stargate SG-1 (but only through season 8 - the Ori just weren't believable as "bad guys" compared to the Goa'uld, and the show just wasn't the same without the original SG-1 team)
- Buffy
- Star Trek: TNG
There's also classic Doctor Who, which according to FSUZeta doesn't count as a completed series, because of New Who.

To be fair, when Doctor Who went off the air in the 1980s, it wasn't wrapped up as neatly as, say, B5 or TNG, so the new series is really a continuation of the old series - the season numbers started over at 1, but the Doctor was in his ninth form (following his eighth regeneration).
Then there are Quantum Leap, Farscape, Seven Days, The Avengers, The Prisoner, V (the original) ...
And there will always be a soft spot in my heart for Star Trek: TOS. That was the very first science fiction TV series I ever watched... my sci-fi gateway drug