Usually the most difficult as well, but in some games this is not necessarily the case.
Ganon at the end of LoZ: OoT was weak.
A Stalfos is harder than him.
Halo didn't need a final boss. An all-out Covenant assault would have done the trick. But nooo, Bungie had to throw in that albino... thing.
2. The occurrence in which the most difficult class you take is the last one for the day.
"I really feel like going home, but I have to deal with the final boss first."