if Floyd fought Paul Williams right now, who would be the underdog?

Not at welterweight he hasn't, the only truly tough fight he took at WW was against Oscar, and only because Oscar got every advantage (weight, ring size, gloves). Besides that fight he hasn't fought the best welters, he's only fought fights that he knew he could win and make good money off of. If he had fought all the tough fights that he should have, then there wouldn't be any question as to who had a better resume between him a Pacquaio.