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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Humm... Tough choice, I'll have to get back to you on this one after I think it over a little. Thinking Silverchair's Frogstomp, but that's more mid 90's.