I have always like Claypool, but I would have to say Cliff Burton. What he achieved in his short life was incredible, and to me, was the most musical out of all the bassists I've. Someone told me that Sheehan was the best ever, but he just plays superfast without really feeling anything (at least that's what I hear in his playing). Simply put, the 1st album, with his recorded solo, says 'bass solo, take one.' and to this day is one of best I've ever heard.