Don't know if anyone mentioned him yet.. but Gary Moore... No contest IMO...
yeah, he's the guy all your guitar heroes mention if you ask 'them' this question....
RIP Gary.. This world is a less rockin' place without you....
I'm not knocking one of the best rock/blues players, but this is about BANDS. This disqualifies a lot of great artists who were solo acts.
This is a hard topic because there are so many bands who were awesome for their own thing. One may be fantastic at songwriting, another for passion, another for fun...
I think the one thing they all have in common is that they were greatest when they were young. I think growing old limits your ability to create and express yourself. You start wanting to preach ideals and can't seem to "let go" like you could when you were young.
or maybe it's because they are usually off drugs by that time...