All ones prior to this are excellent also, but these 2 feel the most "well-rounded" to me. Not without flaws of course, but they just feel more complete in what they set out to do.
VIII's got huge polish, but combat is very divisive, and I can't say I'm a fan of magic spells as stocked items. Plus Limit Breaks and the Aura spell were so over-powered and abusable. Were actually very few towns/cities, world map felt a bit empty. Story got a bit surreal around Esthar. Shame, I think it's by far the best soundtrack.
VII is fun, but extremely rough around the edges in a lot of departments.
VI is great, deserves most of its praise but there are some seriously boring stretches in that game, and there might as well not be any random encounters for the first half of the game as they're so pathetic when you have Edgar and Sabin's spammable, high-power, no-cost abilities, and Shadow's 30 gil-a-pop Shurikens. I didn't like the World of Ruin stuff that much. More over-rated than VII if you ask me.
I could keep working backwards, but my point is - IX and X feel the most whole and balanced to me. In every aspect. Pacing's still weird in places, but they feel the closest to perfection to me. IX has the most developed villain by far. Striking existential themes, very colourful characters, many locations, simple but fun ability system, a very varied story that has roots in old FFs but grows into something very grand.
X is like XIII done right. Somewhat linear, but the characters and lore get so much out of it. Lots of juicy end-game content, the most challenging optional fights in the series, excellent voice acting (no, it's not perfect all the way through but as a whole, and for its time, this was an incredible job). I think it's very hard to forget Spira.
Worst: XIII, II.
I'd rather say "least-good" though. And I went with the 2 most popular picks for it. Firtsly: XII is NOT a bad game...it's very different, and the story's pants but everything else - especially the soundtrack is offensively under-rated. Now that's out of the way: II. It's old, and is a nice early testament to how Square tried to keep the systems fresh and changing. But it made the game very weird and grindy in places. For being good at the game and taking minimal damage, you suffer later for having lower HP pools? Mneh.
XIII is a good game. It's not _great_. It's not at the quality of other FF's, and without that name on the cover I think people would be less harsh. That said, while the linearity and "9 chapter tutorial" serve the story well enough, it does get frightfully boring outside of battles. And while I think the "you only hit X to auto attack everything" part is exaggerated, they're only fun for so long when you get free reign over your characters paradigms. An example of it being at its best is probably the Lightning Vs Caius DLC in XIII-2. The story wasn't bad, but datalogs bogged it down so much, and while I liked the overall lore - towards the end it got pretentiously ambiguous, and came down to too much deus ex machina.
Side note: X-2 is a great game. --- Dojo, casino, it's all in the mind.