Oh now, there's a question without a single answer... but I do have one - but before we get to it I'll run through a few of the other answers that I could have given.

Joss Whedon - I loved Buffy and Firefly, but only bits of Angel and Dollhouse so I might have shrugged off the Joss aspect as him getting lucky - but I can't argue with the fact that most of these highly rated episodes and were written by him. He knew what he was doing with Buffy and Hazel suggests that the problems with later seasons may have been more about him taking his eye of the ball than anything else.

The rest of the writers - I'm not one to really notice who wrote what but I can't argue with the fact that a lot of great stories didn't have Mr Whedon's name next to them. Most of the writers seemed to get where to take Buffy... although some got it better than others.

The cast - not too much to say but I loved all the major characters at some point (even Riley - he was pretty sweet early on) and while the writers wrote them the cast made them real so a big thumbs up there.

The story. A slayer and a hellmouth... perfect...

Which brings me to what I think made Buffy so great and the answer is right there in the question... Buffy made Buffy so great... the person who created her, the writers who wrote her and, perhaps easily forgotten but so very important, the actor who turned her into something so real, so sympathetic, so believable, so rock hard, so cool...

What made Buffy so great was Sarah Michelle Gellar...









... and we're done... a big thank you to Screencap Paradise who made this so much easier than it would have been (in fact I probably wouldn't have made it to 30 without it).