Fortinbras at the end of the 4th Onimusha. I've played three of those now and they never have a final boss that isn't both obnoxious and boring at the same time, but Fortinbras blows the rest out of the water in this department.
Like most of Dawn of Dream's bosses, he has about 3 lines he repeats ad nauseum to help ruin a semi-cool remix of the game's 'danger' theme. And you're going to hear them a lot, because he's only vulnerable for a short period after about a minute of helplessly dodging his attacks repeated over and over. None of your special attacks or Omnimusha rages can help deplete his hideously long health bar any faster than just slashing away like a haymaker when he's down. Have a look at FAQs pertaining to him- every single one makes mention of a period about halfway through the battle until the 5/6 point (so about eight to ten rounds of knocking him down) where he will stagger his lasers and meteors, making them impossible to dodge (and did I mention none of this crap can be guarded against and you can't jump?!), one FAQ referring to this as his 'bs mode'. If I hadn't brought lots and lots of health items I'd have been stuck there forever, and this is on Medium setting.
Much like Necron of Final Fantasy fame (and sort of like his namesake, heh) Fortinbras comes out of nowhere with exactly one appearance and mention before it, after you've beaten what seems to be the final boss (see TV Tropes' 'Giant Space Flea From Nowhere' article). Maybe there might be another Onimusha after that one (it IS Capcom after all) but thanks to Fortinbras and Nobunaga I won't be playing it.