'Buffy' is totally in another level: storyline, plot, screenplay, lines, actors, budget.
Plus the last seasons were aiming also to a public more mature, the show really grown up with time becoming different and always great.
'Charmed' is still iconic and remembered but it was a totally different project, more "teen" and kids oriented, plus every episode had the case and the whole plot was always very lame and small.
Buffy changed the teen supernatural genre and its impact is still seen today (Teen Wolf). The show is on another level. There would have been no Charmed without Buffy!
Charmed is definitely remembered, and though it's a guilty pleasure train wreck it does have a massive following.