Penance for me, but to be fair I haven't fought that many that are on the list. Penance was hard, but it was the right kind of difficulty. Come prepared, know everything there is to know, plan and count your moves accordingly, execute correctly and you'll most likely succeed. Fail in one of those areas and your chances are reduced greatly. Almost nothing in this battle is left to chance, which also indicates the difficulty is well thought out. We don't count Zanmato, of course. I liked fighting Penance.