For me it's longevity. A great band can amaze my brain to tiny little bits for maybe a couple years if they have a small discography. But a band that can keep challenging themselves and putting out great material for 30 years can amaze my brain to tiny little bits for a decade or more. A lot of bands can capture that immutable magic for one or two records, but the artists who can do it for 10 or 15 albums are truly in a league of their own.
Sure, Nirvana and Hendrix had this unbelievable impact and their work is unfathomably good. But who knows if that's all the good work they ever would have done. I find it truly, genuinely mind boggling how someone who grew up worshiping Roy Orbison can out-noise Sonic Youth in 1990 as was the case with Neil Young, or how a staple of 70s rock can craft much of their biggest and most-beloved music in '89 and '93 as is the case with Tom Petty. That impresses me more.