Not sure if this counts, but I'd say Roy Wood. Very well known of course, mainly for looking weird and a certain festive hit, but criminally under-rated as a musician. The Move were great, Blackberry Way has to be one of the greatest songs of the Sixties. The first Electric Light Orchestra album was amazing - don't let the bands reputation put you off, he left ELO part way through the second album, but his influence was still there and it's probably a better, or at least more accesible album. The first Wizzard album, Wizzard's Brew, is also well worth getting. Who else would start a hit record with a Spitfire fly past (See My Baby Jive) or make a pop record based on a marching band (sounds terrible on paper, amazing on the stereo). His solo album Boulders is also well worth getting. A proper solo album, not only did he write all the tracks, but he played all the instruments (and there's a lot of them) and even designed the sleave.
The man's a genius, but gets none of the recogintion of his comtempories like Bolan and Bowie or even Slade.
Here's a couple of his best songs minus Spitfire:
