I am a little afraid to advance this opinion, but:
I disagree with the prevailing sentiment in this thread.
Gulp.
Here’s the thing: you’re going to hear a lot of people say that Firefly was a better series than Buffy, and I’m going to take issue with this. Firefly was a tremendously produced package of fewer than 20 episodes. Through no fault of the creators, it died before it had to even really be a series. It didn’t last long enough to produce inevitable stinker episodes, which it would have. It didn’t last long enough to hook up Mal with Inara and Simon with Kaylee, and then deal with the dramatic consequences of those plot developments in an interesting and original way (or not). Most importantly, it didn’t ever have to go anywhere. It didn’t ever have to give answer to any of the questions it so artfully raised, resolve any of the dilemmas it introduced. It was good, yes, but it was incomplete, so it will always be remembered as a little better than it actually was. Greatness is about the whole story, and the whole story of Firefly is still untold. Can I say for certain that the resolutions to all the plot lines would have unfolded as artfully as the introductions to them did? I can’t. Ask Stephen King: it’s a lot easier to create and sustain a great story with interesting characters than to bring those characters somewhere and make them do something interesting.
I mean, think about it: if Lost had gone off the air after 17 episodes, people would have been calling it the greatest television show of all time. Eventually, though, all of that character development and exposition had to go somewhere, and then the cracks started to show; now the reception is more mixed.
That’s the thing about Buffy. It lasted seven seasons. It told a complete story (and I much prefer the ending of Season 7, by the way, to a series that ended with Season 5; “the hero dies to save the world” is so cliche as to be painful, and that was never the way of that show). 150 hours of TV. Some of them were dreadful. But on balance, that series did things that were remarkable. How often do you hear the old trope that if you set up romantic tension between your leads, and then hook them up, you’ll lose the momentum of your show? How many other series can you name where the lead couple finally falling into bed actually made the show a hundred times more interesting? How well were each of those characters drawn by the end, with how many shades? Rev. Book could have been as well-developed a character as Giles, but he never got a chance to be and so he wasn’t.
Buffy all the way. I’d do Season 3, because Season 3 had the Mayor, who ruled, and Season 3 had Faith (mmmm… Faith), and Season 3 had Jonathan’s speech at the Prom. I wouldn’t argue with 2 or 4, though.