THERE ARE A COUPLE OF THOSE, but I think which one is the immovable object tends to differ from player to player - I know, for me, That One Boss didn't come until incredibly late in the game (although due to GII's aforementioned absolute lack of unnecessary noodling around, by modern RPG standards GII is actually quite short - you can beat it in 50 hours or less - so "late" may be up to interpretation), but there were a few in the earlier parts of the game that could easily have qualified if I had just been playing the system a little bit differently. I am pretty sure the Eye of Valmar probably should have qualified as That One Boss, but the entire plot arc surrounding it justified it being a pain in the ass and made wanting to beat it such a personally important goal that I never found its difficulty cheap.

I find the PS2 version incredibly ugly compared to the DC version, but I am incredibly biased - and I am down with anything that might get more people to try it and love it.