It's also hard to define greatest, in the sense that do you mean are they our generation's Elvis? Or our generation's Beatles, Led Zeppelin, or Rolling Stones? The latter of whose influence and greatness is probably widely debatable?  And can our generation even have something seminal like that?
I am thinking of greatness in a sense of popularity + musical quality (or percieved musical quality based on critical repsonse) + staying power. And I don't listen to what people would consider "great" bands by those standards, so I have a hard time coming up with alternatives.