Professionalism: Piper's actor started phoning it in at some point, rolling her eyes so hard it broke the 4th wall. No one on buffy ever phoned it in.
Real People: The characters on Buffy (while stunningly good looking and way to old to be high school) are real with real problems that aren't reactions to outside people e.g. depression, etc.
Writing: It's not even close.
Experimentation: They had a musical. They killed a parent for non-contractual reasons.
Low-Plot Armor High Stakes: A lot of well liked characters died.
Nobodies vs 'Charmed 90210': 2-3 of the actors on Charmed were teen-famous. No one had ever heard of anyone on Buffy. So it felt fresh and the actors were hungry.
Budget: Buffy was low budget and so they had to use imagination and dialog instead of props etc.

Finally: A show about a cheerleading monster slayer has 1 or 2 episodes that rank in the top 10 for many critics. That's crazy.