First of all I'd like to say Nirvana is one of my faviourite bands, but I honestly cannot say Kurt was a great guitarist, he was crap but honest about it, and that doesn't necierily mean his music is crap... I mean Dylan's stuff is simple but it really gets you... I'm his abbility in song writting no composing 3 minute solos....
Secondly, when I used to just listen to bands like Greenday and Nirvana I thought it was amazing and ground-breaking (i mean to me it was)... Then a friend gave some Black Sabbath / Led Zepplin albums (both of which I cannot stomach, except for their work on the acoustic which is absoloutly buitiful), and I eventaully got my hands on some KISS and WASPS etc, and I reilized how derived the sound was... This really doesn't change the fact that "heart shaped box" is a good song...
I'll back Skee1, and say there is no such thing as a "best band ever", as at the end of the day music comes down to taste, or else we'd all be playing the same thing, and thank god we dont... Imagine how boring that would be...
Keep you ears open, because the intresting stuff that's happening isn't really in guitar-though a pedal- through an amp type music, It's stuff like Sigor Ros (yes there is a mild resemblance to Pink Floyd) and Aphex twin...