Who do you nominate? What about if we open the field to all the justices who have ever sat on the Court -- who wins (and who loses) then?
Also, in the spirit of our companion blog, Moneylaw, let me suggest an additional category: most unlikely to serve. In this category, I nominate James Wilson. Justice Wilson was born in Scotland and attended three different universities there without managing to earn a degree. Despite that (or perhaps because of it) he emigrated to the United States, passed the Philadelphia bar, and started a very successful law practice. He apparently also engaged in land speculation, illegal hoarding of essential goods, and excessive gambling. But he was a good revolutionary and quite a bright fellow, so President Washington gave him a position on the Supreme Court. He served for nine years - in part from a debtors prison when he could no longer avoid repercussions from his financial shenanigans.