I'll answer this two ways:

In terms of PFP ranking, then Hatton is probably tied with Diego Corrales as far as having the highest PFP ranking of any Floyd opponent. Corrales was undefeated and considered a top 7 or 8 PFP fighter when he fought Floyd, much like Hatton is now. (It's easy to forget now that a TON of people picked Corrales to beat Floyd). I'd have to put Hatton above Corrales because Hatton is in great shape and is focused for this fight, whereas Corrales was weight-drained at 130 and facing legal trouble at the time.

In terms of toughest matchup, I'd have to rank DLH as Floyd's toughest opponent, even if he wasn't ranked as high PFP as Ricky is right now. Oscar had the one weapon (a good jab) that could really bother Floyd, and he had a big weight advantage, which Ricky won't have.