In all of the 165 pages this thread so far accounts for, I haven't seen the boss I found hardest mentioned as much as a single time, whilst several bosses I can't find any great challenge in have had their names nervously, possibly even wrathfully chanted.
Gruntilda, from Banjo-Kazooie. Hands. Freaking. Down.
It takes a very, very long time, because you can't just attack the damn witch whenever you please; you have to wait until she stops, until statues appear, until...etc. And when those windows of opportunity are open, you'd better not fail to use them or you'll be sorry for yet another tw