I wouldn't say "impossible" but there are few bosses that were challenging. The hardest one was Ozma from FF IX, I did manage to beat him but I had really hard time doing it. Even tho all my characters were on high level and I learned all abilities I still had problems beating him. But after many tries, and staying calm I managed to beat him but I must say that "luck" was playing main role in this battle.
As for Emerald and Ruby, I really didn't find them that hard, yes I had few problems but I handled them pretty quick. But I must also say I was surprised that they were a way bigger challenge than Sephiroth.
I'm also surprised you didn't put Omega Weapon from FF VIII in the poll. He was a quite a challenge for those who didn't used Holy Wars or Hero, for those who wanted a real battle. I really enjoyed fighting him because he was much harder then the last boss Ultemecia.
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