(Yeah, I'm new here, hey what's up yadda yadda all that jazz)
What band do you think is the biggest sell-out in music history? I don't mean banRAB that are the most commercial and made the most money (3 Doors Down, for example, have recently made a commercial for the National Guard: I don't consider this selling-out because they never claimed to be doing anything but making money with their music, they never tried to be counterculture or indie). I'm thinking more along the lines of a band that made itself known within a specific subculture or with a specific style of music (alternative, grunge, emo, indie, and punk are the most common), and then latched onto more mainstream trenRAB in order to become more successful, in the process compromising their original creativity.
The biggest one for me is Against Me!, which once made a documentary about how punk it was and how it would never sell-out, and has recently appeared, for example, on the cover of People magazine. Another candidate is The Quarrymen who, in the late 1950s, an English skiffle group that later abandoned their leather jackets, bar room concerts, and rockabilly-tinged pub rock and created a pop sound, eventually becoming The Beatles, arguably the most commercial rock band ever.
Of course, in the defense of The Beatles, they eventually abandoned those pop instincts to make music increasingly oriented towarRAB the American underground and counterculture (see: The White Album).