It is a very good show, and is one of the few shows about vampires that I can think of. While it was more about vampire hunters than vampires there were certianly a lot of great elements to the show, and it was a fun way to explore the nature of good and evil.
Yes, Buffy is the greatest vampire show of all time because every vampire show that has come after it is only copying Buffy. Buffy started a revolution and was very far ahead of its time. Now vampire shows are simply trying to get in on the success of those before them.
It covers so much and the Buffyverse is awesome. It has characters that spawned spin offs, it has great acting and writing and captures a lot of what it's like to be an outsider. It deals with friendships, loss, popularity, drinking etc. It's such a great show and although it's the best vampire show ever, it's more than just a vampire show.
While "Buffy the Vampire Slayer" has a large fan base and is a very popular series based on the mythos of vampires, this series does not do the actual mythology of vampires justice. Every fight looks staged and the vampire abilities portrayed barely scrape the surface of what a mythological vampire was credited with being able to do. It's truly unbelievable. Entertaining, yes, but the greatest vampire show of all times? No, there are many others out there that are more deserving of that title.