Fortinbras from Onimusha Dawn of Dreams, in both his genma and human form, challenging and epic.
Final boss of Shadow of the Colossus. I mean, this guy is HUGE.
Or maybe the final boss from The Suffering and the final boss from The Suffering: Ties That Bind. It's not that they were huge or difficult, but because they were symbolic.
In The Suffering, T first faces the two forms of himself, one as a human, and one as his insane beast. Right before the final showdown, depending on whether you were good bad or neutral, your family gives you a few words, and it was time for you to face the anger inside.
Which is similar in The Suffering: Ties That Bind. The villian throughout the whole game, Blackmoore, is the final boss and turns out to actually be your alternate personality. You either get turned evil if you were bad, get rid of him if you were good, or the ending that stays truest to the title - he couldn't change you, and you couldn't get rid of him. It's the ties that bind.
Rest in peace Heltor. God bless you.