To me, The Pumpkins put out 4 brilliant albums basically one after the other and that's enough to put them atop my list. Pearl Jam has been the band with the most longevity. Nirvana and AIC both come with a large "what if" factor though.
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Nirvana kinda brought rock to what it is...i said kinda there. Plus their MTV unplugged in New York to me is probably the single greatest album ever