Big Star. My god, I can't believe this band didn't explode in the 70s. They wrote intelligent rock in the vein of the Beatles (without ripping them off), with insanely good hooks. They were fronted by the former member of The Box Tops, Alex Chilton, who had a huge hit in his song "The Letter" before splitting with them and forming the much superior Big Star.
For an example of their genius, use your favorite p2p program to try to find the songs The Ballad of El Goodo, September Gurls (yes, with a u), Thirteen, Blue Moon, and Kangaroo. I found them easily with Kazaa. Everyone of 'em a pop rock masterpiece. (Warning: If you don't like The Beatles, you probably won't care much for Big Star)
These guys are sort of like the Velvet Underground in how they had very little in record sales, but they inspired many great bands who came after them, like The Replacements and R.E.M. The Replacements even wrote a song about Alex Chilton for their album, Pleased to Meet Me.
They had barely put out their second album when they called it quits. The other main songwriter, Chris Bell, put out a solo album that was somehow even less heard Than the Big Star albums, but has another one of my favorite all-time songs "You and Your Sister", which Kim Deal covered on the great This Mortal Coil ablum "Blood". Sadly, Chris Bell died in a car wreck in 1978, another brilliant artist to remain forever unknown.
I could name dozens more great bands overlooked by time, but this is already too long and I'm sure to get flamed enough as is.
