Stargate SG-1, Stargate Atlantis and Stargate Universe are probably the most special to me. Not sure if they're the best necessarily, but they're definitely the ones that had the biggest impact on me.
I tried rewatching Voyager a while ago but it was so cringeworthy, I just couldn't continue.There's really no doubt that Star Trek is the best. And popularity would overwhelming agree! It's great to start with the original but you can skip it if you want and start with The Next Generation and keep going in order with Deep Space 9, and Voyager. You can skip Enterprise, but it's okay.
If you're that old, how are you not listing off Land of the Lost!? Yeah yeah, Chacka never did get eaten by Alice, but still!
I really wish Firefly had gotten a chance to get another couple seasons. I think a 3-4 season arc would have been truly revolutionary TV. Very grateful they got the Serenity film though (which I loved, but also saw before seeing the TV show).
My favorite is Star Trek, both original series and the Next Generation. The original series was just as much a horror show as it was sci-fi, which gave it an additional layer.
I really enjoyed the show Defiance on Syfy. Painfully underrated series. Fringe was also pretty great until that last season, which basically ruined the show for me. It really sucks when a show goes so far off the rails that it takes it's legacy with it. *cough*Lost*cough*.
For me it was the episode with the little guy on the wheelie cart, he hitched ride...in people.