II and III both have fairly simple characters, but they're memorable enough for characters from an NES game. I would say Guy is definite weak point of II, Frioniel and Maria have some fun personality in their lines across the game, but Guy is hard carried by "Guy speak beaver" (which isn't even a literal translation, in Japanese it's "Guy speak with animals", the English localization made it sound a lot funnier by specifying beavers).
IV's character are fairly two-dimensional, but they have enough charm as well as character arc playing into the game's over theme of revenge, guilt, and forgiveness that they're the earliest cast you'll often see people have an emotional attachment to. If there's a weak member of the cast, I'd say it's Rosa since she's the only one with no conflict or trauma specific to her. She's just there to support Cecil and be what makes Cain jealous of him. She's fairly typical of her character archetype, although I do think it's a bit unique how her and Cecil are already together before the game even starts. She's carried by her scene before the final dungeon where Cecil tries to leave her behind and she sneaks back onto the ship with Rydia, proving she has backbone if Cecil is doing something reckless.
On a side not, most of the new characters in The After Years are duds, but I do like Ursula and Leonora. I also liked how Opera Omnia had Leonora redeem not one but two villains from other FF games by the power of being really, really nice.
V's cast also isn't deep, but they're have fun dynamics which works with the game's overall tone. I like how they all embody one of the four elements in a way. I think something that helps them is how there's only 5 of them, and technically just 4 at a time. Because of that they're a tight-knit group and it's easy to see the character dynamics between any given pair.
VI is the first game where people would more likely describe characters as three-dimensional or deep or some such praise. It's a huge cast, but they're mostly a pretty memorable crew who each have their own character arc or backstory that ties into the game's theme about coping with and recovering from tragedy. Umaro and Gogo are weak links, but they're gimmicky bonus character you can optionally recruit late in the game, so everyone's pretty much fine with that. I think Gau also kind of weak, his feral child thing means there's not much to him outside his short personal quest in the second half. For also being an optional character, I do like Mog. Him losing all his Moogle friends and you finding him alone in the World of Ruin pulls at my heartstrings every time. Character dynamics are a bit all over the place because of how big the cast is, so they're kind of separated into groups within the overall party, but something I like is that Square designed the game so you're forced to use everyone for at least a little while, which forces the player to imprint at least a little bit onto everyone.
VII is my favorite FF, so obviously I think it has the best characters. I like all of them and wouldn't say any are a weak link. Obviously it's Cloud's story at the end of the day, so some characters get more importance then others, like Tifa, but I think everyone contributes well to the plot and has a nice personal tale. Even Vincent, who's optional and has no large scale quest like Yuffie, feels like he adds something meaningful to the plot and fits in with the group. Maybe I'm partially saying this because of later spin-off material, but I feel like you can easily imagine the group dynamics between anyone in the party, even the ones who barely or never directly interact with each other. The one thing is that didn't really do like VI and make you use everyone at some point. Mainly Cait Sith is never needed to be used at any point.
VIII is a weird one because it's the most lop-sided in giving all the focus on Squall and Rinoa, with the rest on the side supporting them. Because of that, a lot of people tend to be dismissive of the other characters as not important, but I think the creators mostly accoplished what they intended. Without getting into Squall and Rinoa, I think even with them as the big focus the other characters still have their places to shine. I like how Zell has this low-key thing throughout much of the game where he feels guilty for fucking up that one time and seemingly getting Seifer killed, so he keeps trying to find ways to redeem himself and practically make himself Squall's second-in-command. And I find Irvine funny because he's the type of character you'll basically never see anyone say is their favorite, but he's explicitely meant to be forgetable in-universe, which is why he tries acting like a show-off when you first recruit him. It's funny because when you first play the game you might think the devs intend for you to find his too-cool-for-school antics cool, but if you paid attention and replay the game it's clear he's actually a tryhard dork. Even so, as things go he says some things later that show he's the most mentally stable of the group in the end, though one of his best scenes is entire optional and extremely easy to miss. Selphie is notable to me she acts a lot like autistic girls I've seen or met IRL, which isn't something I've really seen elsewhere in fiction. Overall, I think VIII's cast is actually the most "real" in a way, which I think might be exactly why some people get really frustrated with them. Because they're dorky and frustrating in way actual dipshit teens are.
IX is my second-favorite FF, so it's also the game I think has the second-best cast. I like all these weirdos and I think the developers did what they intended in giving each one a story and angle that fits within the game's overall themes. They also have a lot of fun interactions between them. There's some lop-sidedness with focus given to Zidane, Vivi, and Garnet over the rest, but I think don't any of the others are ever irrelevent or could be written out without negatively affecting things. But hot take, if there's a weak member of the group, I'd say it's actually Garnet. She plays the slightly rebellious princess archetype too closely that it starts to hold her back. I feel like her romance with Zidane never really goes beyond the two of them feeling kinship in being adopted orphens who felt out of place. And I think she has the least dynamics with the rest of the party, sans maybe Quina but I think Quina is fine because of how their comic relief character. Like Zidane has clear relationships with everyone, no need to go into it. Vivi has Steiner making him his little buddy, him and Eiko have this thing where she drags him into hijinks because she sees him as a kid her age, and him and Eiko get watched over by Freija at times (in Japanese they literally called her onee-san). Freija has her moments with Salamander, and he also has stuff with Eiko where she tries screwing with him and he tries ignoring her but gets her out of trouble in a tsundere way. The main missing links here are that Vivi and Salamander never interact I don't think, and Steiner never really gets a chance to interact directly with Freija or Salamander. Garnet, despite being the main heroine, only really has her relationships with Zidane and Steiner, and the one-sided rivalry with Eiko, but she doesn't particularly interact with anyone else. Even her interactions with Vivi are kind of through Zidane. Hmm, we could probably think over and argue on this a lot more, so I'll move on.
X has a solid cast who all contribute meaninfully to the story. Probably helps there's only 7 of them. Kimahri is a weak link because of how little he talks, he doesn't really have a dynamic with anyone except Yuna and a little bit Tidus, but he doesn't a couple cool things and I don't think the story would improve if you wrote him out. We've already been talking about X so I don't have anything more to say here.
XII has the weakest cast since like, the NES games, I think. I like a lot of XII's plot, but the party on their own are kind of iffy. And I think there's kind of a confluence of issues as to why. For one thing, XII barely has any sort of comic relief scenes, or really any scenes of the party just like, chilling for a little bit. Everything is really serious and plot-important all the time, so it's hard to get a read on if the characters have any relationship between each other outside of needing to travel and fight together purely because of circumstance. I think I remember Ashe saying something implying she considers Vaan and Panelo friends in the ending, but that feels weird because I can't imagine what exactly she thinks of them beyond that they were reliable to fight alongside. Vaan does have a thing with Ashe in the game, but it's really that friendly in an intimate way, it's more that he serves as her moral compass in their whole discussion about revenge because of the Occuria using visions of their dead loved ones to manipulate them, which Vaan didn't fall for. And another thing is Vaan. He's a rather archetypical teenage JRPG lead where he's brash, naive about the world, and doesn't get much respect from older party members at the start. You could broadly compare him to Tidus, but the thing with that is that Tidus only really acts in cringy ways early in the story. As the story goes on, Tidus starts acting in more level-headed ways about things and you can feel earns more respect from the other party members, to where it doesn't feel weird that he's the one emotionally leading the charge into battle at the end. Vaan though never feels like he's anything more than a dorky kid to most of the rest of the party even to the end of the game. Well okay, Balflear respects him enough by the end to trust him to look after his ship for a while and then invite him to do air pirate stuff, but it's not quite the same thing. I think that's why so many people don't like Vaan as the nominal protagonist of the game. And speaking of character dynamics, I don't recall Bashe really talking to anyone 1-to-1 except maybe Ashe, now Panelo with anyone except Vaan. It makes sense that Balflear was sort of the popular character people often say should be the lead, since even outside running gag of calling himself the leading man, he does kind of feel like it in some ways with his interactions and being the one with the airship, but the story is written to where it makes the most sense with Vaan as the viewpoint character to start so we can see a ground level view of politics in Rabanastre before the plot really kicks off. None of the characters are bad indivitually and they all contribute something to the plot, but they're less than the some of their parts as a group because there's little effort put into making them feel like they have any particular comradery.
