I still think the water palace in FF1 is the most gigantic thing in the world, I have no idea how I find my way through it, maybe its either that or it just looks big to me, haha.

FF2's Jade Passage -> Pandaemonium trip was literally a trip to hell with no way back, that was quite the travel. FF3s Styx Tower -> Dark World was probably a bit shorter but even worse, due to the Dark World random battles/boss marathons.

I think in general, it seems like the more recent the FF games get, the more straightforward their final dungeons turn out to be.