The first question is like asking 'Who is the greatest scientist ever?' in that you will never, EVER, get a definitive answer. Almost everyone has brought something new to the table, and almost everyone has also taken some level of inspiration from someone who came before them. My personal favorite is Dave Groll, because he was part of the band that pretty much defined my favorite flavour of rock music and has since proven himself to be multi-talented as a drummer, guitarist, and singer. But trying to argue that anyone is the greatest is UTTERLY, UTTERLY POINTLESS!
(the answer to the second is Lex Luthor)