While I love Leroy Sibbles, and his abilities makes him qualifiy on all accounts - he wrote songs that have become standars in the genre, he sang lead on countless hits, and he was one of the most in demand session bass players in a genre where the bass is king - he is hardly well known outside Reggae circles, which unfortunately means he is really not known at all... I think Jack Bruce should definitely be considered. He often tops lists of Rock bass players, but he was as good as a songwriter, especially from a technical stand point, he had studied composition and his writing was quite advanced and he wrote in many genres, maybe not as much from a commercial stand point, but is writing hit songs the only parameter? He did write a few hits that are world famous and he also wrote songs that were covered by others. His singing was also very advanced and expressionistic, he used it to almost the same extent as his bass to express himself, I could easliy have seen him have sucess as a singer alone. He didn't really sing too much material that was not his own, except a few blues covers, so it's only speculation how well know he would have been as a singer without his songwriting. The fact that he is not really a world famous (household) name of course speaks against him, but that wasn't really the question, was it?