I've seen almost every episode of Trek produced and own most of it on DVD. Oddly I find TOS the hardest to watch and I still haven't seen half the episodes. The 1960s-era pacing really grates on me when the episodes aren't outstanding. I love the TOS movies though. TNG was the series I grew up with and it's still my favorite, though DS9 is probably better objectively. Taking production values of the time and relative quality into consideration ENT is probably the worst series, but I still find it enjoyable.
I watched all of Babylon 5 recently and I think it's good but not better than Trek. I wasn't as impressed with the writing as a lot of people seem to have been. For the most part it was the typical heroes journey / chosen one stuff that's been done many times before. I think watching it over a decade after it first aired might have hurt my impression of it a bit.
I also watched Farscape recently I think it's great, easily as good as any Trek series, though the first season is pretty unremarkable and it doesn't become consistently excellent until the third season. The journey is well worth getting there though.
I love the X-Files, it might be my favorite series or very nearly so, though it jumps the shark somewhere around Season 6.
I'm also a huge Lost fan, though how I feel about the series as a whole is going to depend on how it ends. I think this is one of those shows that if the ending is bad enough it could undermine the previous seasons. I'd like to think the producers aren't that incompetent, however.
I've never watched Doctor Who or the new BSG (and god, I've been spoiled to death on it on this forum so kind of don't want to watch it now), and I'm just now getting caught up on Fringe which is good but not great at this point, about 1/4th into Season one.
I liked all the early 00s invasion shows and wish at least one of them had stuck around but Invasion was the one with the most potential I think, though it started the slowest.
I've probably forgot a SF/F show here or there but that about covers it I think.
