I'd say content that exceeds the final boss in terms of difficulty is a pretty clear nominator.

Either of the Spira entries for me. They really pushed the boundaries in terms of postgame content, FFX allowing you to face off against so many unique bosses and challenges, and eventually if you want to grind out your stats, facing off against absolutely mad bosses as well. FFX's postgame mainly fails to me in how grindy it is, which is why I don't think I could play it on a different version than the PC version nowadays which features 4x speedup and SMT Rush Mode.

FFX-2 doesn't have quite as expansive a postgame as most endgame content more closely matches Vegnagun in dfifficulty, but it does have what I consider to be the single best postgame dungeon in the series, offering a challenge you straight up won't get anywhere else in the series, and I love it.

FFXII does have a real good postgame as well, including deeper parts to several dungeons scaling them up to postgame difficulty was a real nice call.

None of the pre-X entries come even close for me though (at least the ones I've played), some like the PSX era even having just one or two bosses or something.