The premises are:

1. There are varying levels of quality in the FF series, i.e., they are not all the same.
2. Quality is subjective in nature (or, one man's trash is another man's treasure).
3. Subjectivity and Objectivity are mutually exclusive.
Therefore,
4. There is no basis for defining a level of quality that can be universally accepted as the "Best" or "Worst."

That is how you construct a proof.

Now, here's my opinion.

8 is not the worst at everything. As trite and sloppy and dimwitted as the story tends to be, it is a journey that gets interesting if ONLY because the world is so lovingly crafted. Not just the looks! But the people, the places, the things. They all have a clear mark of craftsmanship, and range (very unusually for the series) from contemporary to a kind of bizarre mystic/sci-fi cocktail.

The characters, while one-dimensional doorknobs, are at least realistic-looking. You can connect with them on a subconscious level that was impossible in any previous FF. The music is excellent, and comic relief is at a pithy, self-aware level that has never been equalled. The gameplay is fun. It's fun! Limit Breaks that let you PARTICIPATE in the fighting! Wowzers!

And as ham handed as the love story is, can you name another FF that did it better? 9 comes close, 10 ekes it out by a fair margin, perhaps, 7's was a little weird for me, but (and I think I'm tipping my hand here), 2's fails in THE most spectacular way possible.

I may as well stop beating round the bush:

No, 8 is by no means the worst. Not even close. If you were sufficiently bored, you'd play it. 13 also is not the worst, though I'll say I think it's close. The sheer look and playability of the game (repetitive though it may be) put it just a notch above 2 in my book.

Final Fantasy 2 is a stinking, smoldering turd. It's ugly, slow, frustrating, sounds inferior to the first game, and has the mother of all bad RPG stories. I've played it through to the end, and it's just so devoid of the drama it pretends to have that it isn't even worth laughing at. None of the characters is the least bit multi-faceted and none of the events seem to really mean anything to them. The writing is just obligatory and dull. Trope after anvilicious trope. If this thing was supposed to be Star Wars in a fantasy setting, it failed miserably. At no time are we given more than the most perfunctory of glances into the 3 mains' inner lives. The supporting cast is forgettable, and the villain is of the "Wimpy McDo-Nothing" variety. By the time he shows up (Palamecia), you're like, "Oh, yeah, that guy...?" The controls are stiff, the menu is clunky, the inventory system is a bad practical joke on the player ("Hey Hiryu! Would you mind terribly getting the HELL out of my bag???"), and to top it all off, it had the first appearance of a wangsty character, Prince Gordon. I kill him and leave him dead whenever I find it in my heart to play this game, a feat of enormous generosity on my part, in the rare occurrences where I really am just plain obsessed with playing through the first 6 in order.

Being perfectly honest, my oPINion is that 2 is rock bottom until they make a main-series game out of "stone knives and bearskins."

Even if you hate 13 (and I don't think you'd be wrong to), 2 just sucks in a way that 13 can only dream about.