Ahh as to the first bolded point you made, I suppose the ratings should be the most important criteria. I will remember that for future discussions with you regarding greatest anything, film or television. Clearly Avatar rules them all.
As to the second point, well for my part, Buffy was a great show my ex-wife and I enjoyed together. It had enough of the fantasy/sci-fi element that I liked and appealed to her for other reasons. #3 seems a bit high to be honest, but it is one of my favorite series of all times. The show was funny, had some very dramatic moments, and plenty of good writing.