I'm going to take the oddball choice and go with Final Fantasy XII.
I played 1, 2, 4, 5, 6, 7, 8, 9, 10, 12, and Tactics just to clarify what I have experienced of the series, I really like FFT but I won't consider it in the main line-up.
For me Final Fantasy XII really hit all the right notes for me, after FFVII I realized that the Final Fantasy series has become one of reinvention, and instead of just building off what worked previously, they scrap everything with each release and start over. For a long time I lamented this approach until I played FFXII.
So here's a few bullet points on why I think it's great:
-Gambit System: Once you figure it out and get a few hours into the game to acquire decent gambits, this becomes one of the best ways I've ever seen of setting up auto-battles giving you implicit control over how character AI works. My only lament is that it wasn't more robust but it worked very well and it made grinding a breeze as you just set up your gambits and walk around the landscape letting your team mop up.
-Hunt System: As a fan of extra content and side-quests, the hunt system was a thrill. The ability to hunt increasingly more difficult enemies, the majority of which are boss-level. I love when games give you a reason to backtrack to areas and experience them again and unlock secrets.
-Secrets: Good lord this game has tons of secrets, some are annoying (like how you get the Zodiac Spear) but others are great like finding hidden Espers, secret bosses, hard-to-find hunt club enemies, powerful weapons & armor
-The World Lore: Ivalice is my favorite setting of any Final Fantasy, it's distinctly high fantasy with a hint of steampunk with it's magically powered airships. I love the varied races in the world, and they did a fantastic job of making them feel like they belonged in this world.
-The Bestiary Codex: I loved how not only could you unlock info about any enemy you defeated but defeat a set number and you unlock background lore about the game's world as well.
-Sky Pirate's Den: This is like an achievement system, but it was really fun to play with.
-Being able to switch party members out mid-battle: You could only have 3 guys on screen at once, but as long as someone wasn't taking an action or being attacked you could rotate them out for another party member.
-HUGE open areas: Unlike the more linear games in the series, FFXII's areas were comprised of multiple connected maps each one very large and open, it was really fun to explore and the world felt big and alive. I also loved how lively and crowded the cities felt.
-Very challenging bosses: The main reason you shouldn't rely too heavily on the gambit system; The bosses are tough as nails! Numerous bosses will put all kinds of nasty debuffs on you and have very specific immunities and weaknesses that you can exploit. If you don't set your team up properly you'll end up with a giant circle of everyone healing each other trying to stay alive and nobody attacking the enemy.
-Realistic Loot System: You NEVER get money from killing monsters, you get various items that make sense to those creatures like hides and teeth that you can then sell for money, and even unlock special bazaar packages for selling them in specific quantities.
-The Story: While I don't really care for 2 of the character Vaan and Penelo since they were just kinda thrown in there, I did really enjoy the story, and it is very interesting and politically charged with some really cool moments.
-Connection to other games: FFXII shares the same world as Final Fantasy Tactics although removed several centuries or more, but playing both and finding the connections in the story particularly in the Espers is really interesting!
-Great Battle System: While very quirky and different, the battle system just works. The Gambit system comes together great with everything, you can still give manual command to your characters, enemies are present on the field and you can move around and run away in real space and aren't limited to just menues, there's chain combo attacks that increase the quality of loot that drops, and different enemies actually react to each other's presence and have great AI.
The game is certainly not without it's flaws, the license board while not bad per se ends up making everyone in your party cookie-cutter interchangeable half-way through the game and there's little class distinguishment. There are also some infuriating parts that encourage save scumming and relying on guides to not miss special one of a kind items or hidden bosses.
Still I think in the main series it is my favorite of all of them!