I like the idea of this question...for me, the band of the decade would probably have to be Queens of the Stone Age. I could list many reasons, but really just because of the quality and consistency of their music. Hell, buy the vinyl version of "Lullabies to Paralyze" and you get tracks like "Like a Drug", "Infinity" and "Precious and Grace" and there's no way you can tell me those songs suck.
But from a public/cultural standpoint, I don't think there is a single band or artist that has defined this decade. We're in an era where the nurturing and development of artists is almost extinct. One need to look no further than the dissolving of record labels, the fact that few people sit through entire LPs, and most kids aren't "fans" of a band and instead they download songs. Flip through the iPods of any of the 20-something I work with and you will NOT find more than 4 or 5 songs from a group or artist. There is little value for something that you can get for free.
So that being said, I dunno. Maybe it's Kanye West? Britney Spears? Hannah Montana? What really inspires people these days and sets a benchmark by what an artist or musician should aspire towards? I just don't think that there's anything obvious, certainly nothing that speaks for my own tastes. Even in the 1990's we could have said the decade was summed by anyone from Nirvana to Radiohead to Outkast.