Probably 8 or 5. 8 gets a lot of hate and a lot of it is IMO unwarranted, whereas 5 just doesn't get mentioned much even though it's a very stellar entry.

Honestly the best writing (not story) the franchise has had. 

Y’know I’ve already been interested in giving this game a shot but hesitate due to the lukewarm word of mouth. Hm. Maybe I’ll reconsider… I know a ton of games that are waaay overhated that I end up loving. (Case-and-point: MKWorld and Echoes of Wisdom.)

It's close, but I'm going with the OG Final Fantasy 1. The biggest reason people don't like it is because they want it to be something that it was never capable of being. (The heavy narrative FF games started with 4.) Final Fantasy 1 is a game about resource management. Almost all FF games do this to some extent, but the first one did it the best IMO. Also, I find the NES version to be much more fun than the GBA version, so for some reason the version you play seems to matter.

I almost picked FF 11, but I think the best version of the game really only exists on private servers now (or possibly it just doesn't exist anymore).

I've missed a ton of them, but based on VI, probably not IV or V. I'll go with XIII, mainly because it has great lore and a good story that are supported by great, diverse visuals. I haven't played XVI yet, but XV has a pretty boring world and visual style, and the same goes for XII. I can't speak for XI or XIV, but as far as I've seen, XIII is the last time the series had such a great world, lore, and visuals (I really appreciate the colourful, diverse look of X and XIII). XIII is definitely a flawed game, but I don't think it gets enough credit for the things it did right, and even the criticized parts aren't as bad as people tend to claim (although gameplay is really boring for the first 20-30 hours - that's just inexcusable).

100% I would almost say best story but the narrative and characters were fantastic. 

Definitely give the game a shot. Engage in the world building and read the lore in game as it adds so much context. This game is loaded with narrative and a lot is happening all at once. Multiple (5 Kingdoms) at war and you can read about where every character is and what they are doing in a timeline. You can also read each characters thoughts and what they think of each other individual character. The game is loaded, it's not a game you can just go into and burn through. It's a massive cinematic experience with wild world building and lore with breath taking cinematic that lean on a lot of spectacles. 

I've played 1-9 and of those I'll say FF5.
I do get why: 4 & 6-9 are all story heavy and FF5 is kind of forgettable (especially if you've played 3) and that's why I wouldn't name it among my favorites. But in gameplay it may be best of all.
Also the first two excel in some things.

I'd say at 87 meta, the highest for a mainline FF since 2006, that it was fairly rated. Very high highs but also some of the lowest lows of all FFs.

They should have cut the boring side quest-esque fillers posing as main story content and been content with a shorter experience, since, truth be told, this was a smaller, shorter game than the average FF. Then it would have stood as an all-time great.

13 has an amazing story and setting that they couldn't fully convey due to development issues during the HD transition. Alongside 15 (and the whole Fabula Nova Crystallis thing, really), it definitely has the trophy for biggest wasted potential in the series.