These trainers are the final gym leaders of each of the hardest versions of their respective game.

They represent the penultimate challenge you face before the true final challenge (be it the Elite 4 or Red in the case of the Johto games)

These are their teams and their movelists.

Now I ask you, which best represents the difficulty curve you'd want in a Pokemon game?

A symbol not of your greatest challenge, but the one that opens the gate to it.