Shen Long in bloody roar 2. I could brutalize every other character in that game on the highest setting, but I was never able to beat him. I came damn close one time and never stepped up to the challenge again.
Also, fenrir and bloodbane in valkyrie profile were pretty harsh fights to go into unprepared (which is damn easy to do since you have nowhere to go once you reach them).
the 10th level of the cave of trials in star ocean 2. it's a wall in the way of progress.
Analog's underwear are digital. right now they're set to inverse 1 (I think that means they're on my head).