best artists in the last 15 years

Jennie Devo (rock) is special.

Beth Hart, Gov't Mule, Hans Theesink and Josee Deschenes. For a source of new music check out www.garageband.com. Thousands of Bands representing all genres of music.

Ida and His Name Is Alive are two very goods bands which are alternative/folk/rock music (you get the idea)

however they have been around several years

Red Hot Chili Peppers
Radiohead

and for the greatest rock band of all time.... Tenacious D.

Hands down, Radiohead. Second, Tom Waits. Third, Red Hot Chili Peppers. Concerning the former, don't know of a veteran group that peaked, i.e. put out three straight hit albums, so late in their career.