I would say BLUR. they have a fantastic back catalogue and were successful mainstream as well articlay. they made the cross-over and back successfully and pushed their music forward.
a lot of people say RADIOHEAD but i think a lot of their material since ok computer is a bit overrated, (oh no!) That album was experimental but was also a set of decent songs. So much of what they release now is so experimental (slightly boring!) that it defeats the point of listening to music.