Hm...that's a tough one. I guess I might say the castle where you get the Sunfire because Gordon joined your team and he's super weak for a long time.

It has one of the best tracks in the game playing though so that helps...

I'm not really sure as my mind is blanking on a lot of it.

To answer this topic, I think I'm gonna go ahead with my plan to replay the game and actually finish it this time. I stopped right before the Jade Passage for some insane reason and never completed the game.