10cc

Radiohead by a mile.

Then Arcade Fire and Muse.

For me its gotta be The Cat Empire.

Their uplifting tunes have brought me out of so many ruts in the past few years.

There are several bands that have been great over the decade... The Flaming Lips, Muse, Arcade Fire, etc... But when it comes to hit bands, I'd have to choose The Killers. I adore that band.

Skillet:

They went from virtual unknowns at the beginning of the decade to having an album that debuted at #2 on the Billboard charts.

Skillet made Christian music relevant to mainstream listeners. They're possibly the only band that has songs being played in both churches and bars

Family Force 5:

FFV is the evolution of rapcore. They are the first band of the second wave of rap/rock or rap/hip-hop. Very interesting stuff.

(what other music video has dancing ninjas and breakdancing hearts?)

They only qualify for half the decade, though. I've seen 'em live before, they are great live. They have a 6th member (Xanadu) that is their manager/dancer which is hilarious, live.

Radiohead, Muse, Keane, mmm I dont know who would be the best band, I also like Rise against a lot.

Hoo boy. I'm unsure, really. As much as I want to say Pearl Jam, I can't say it in confidence with some of their studio works that had been released early in the decade (Riot Act).

I'd go with Arcade Fire. It's been quite some time since I've been so impressed with a new group. To add to that, though I haven't seen them, their live performances seem to be quite good.

I'm really quite fond of some others that have already been picked as well, namely Metric and Radiohead.

2000-2009 = The Killers

Eminem or Britney Spears

Rise Against is by far the best band of the decade. I agree with the OP.

Also good this decade were the Offspring, Breaking Benjamin, 3 Days Grace, Streetlight Manifesto, Jimmy Eat World, Springsteen, Foo Fighters, JET, The Killers, Kings of Leon, Pearl Jam, Red Hot Chili Peppers, Weezer, and many many more.