So now that FFVII is effectively finished, it's the right time to go back to comparing FF character building options by the most important metric there is: how good is Blue Magic in this game, anyway.
FFV Blue Magic was very powerful and versatile but suffered from practically requiring a guide to actually get it to work. Even finding which monsters could give you spells would require a lengthy trial and error process, and getting them was a chore without a secondary Beastmaster character. It was an interesting proof of concept, but very, very unrefined.
FFVI Blue Magic was easier to make work since you just needed to have a particular character present in a fight where a Blue spell was used and to not die, but it suffered from attaching Blue Magic to that particular character, who was otherwise unremarkable. If you used him often, you would get some common spells even without a guide, but there was little reason to use him. With everyone getting all other magic, Blue Magic also became a lot less versatile by comparison. Every caster is a buffer/blaster/healer/utility anyway, so aside from a few standout spells (Big Guard, White Wind), Blue Magic didn't stand out that much.
FFVII is something of a return to FFV mechanics with "the character needs to actually be hit by enemy skill to learn it" and the ability to slap the relevant materia on anyone. Though, much like in FFVI, it's going to be just a part of your skillset rather than a full half of it. It's not a regression, though. The indicator of which enemies have learnable skills cuts down on trial and error significantly, and Enemy Skill being just one materia among many means you can keep it on someone without it being a hard choice the way being a Blue Magic or having Learn assigned to a character was. It's probably the first Blue Magic Omicron has actually consistently used in a game, speaking of its usability improvement, and while it's probably less potent overall than the FFV version (much like in FFVI, because you have a bunch of other materia active), it's still a pretty useful skillset to have. The only real flaw in its implementation is the whole "each enemy skill materia is independent" thing, meaning that having two characters with usable enemy skill takes twice as much effort. Though even that could be considered an advantage as it contributes to variety between character skillsets.
As for other, lesser concerns over character building... Well, I think there is no need to speak about FFI and FFIII as they were more rough proofs of concept that would be revisited and polished in later games. FFII was an abject failure, falling down the familiar pits of "learn as you use" RPG systems that encourage you to do counterintuitive things for the sake of rising stats (like beating up each other while enemies deal you zero damage) and making it so some often used skills advance more easily than situationally useful ones. It's also the first game to make characters completely interchangeable (even if they would likely specialize in different things over the course of a playthrough), a trend of FF series that I strongly dislike.
FFIV is pure in its simplicity: every character has unique mechanics, and you have no control over your party, they come and go as plot demands. The disadvantages are obvious as you have zero say in the game's strategic aspect, limited to tactical one, but the advantages is that it makes every character feel unique, connecting them to their in-game role. And, of course, it removes choice paralysis entirely: you may not be able to break the game, but you aren't going to agonize over which job to master and which are good only for their skills.
FFV is, overall, probably the most polished of the complex FF systems so far. Simple in concept (just pick four jobs you like and use their !Abilities), it allows for surprisingly deep customization (barefisted mages! Beastmaster + Blue Mage combo! Rapid Fire Dual Wielding!), which eventually leads into Freelancer/Mime ultimate builds, themselves an improved take on Onion Knight basic concept. It's also sensible enough to give you all the toys to play with halfway through the game, freeing you to experiment and fool around before committing to the endgame. It's not flawless, though: mastering jobs takes more time than optimal, with a lot of them being padded by +X% MP/HP abilities that don't even get inherited by Freelancers/Mimes, and some jobs are duds (nobody uses a Berserker outside of self-imposed challenges). If you do know what you're doing, the endgame somewhat falls apart with everyone running on the same baseline stat-boosting classes plus the best !Abilities, though I don't think it's necessary a flaw: if you've gotten to the endgame, you've earned being absurdly powerful. My personal complaint is that it's another entry into "characters are totally interchangeable" FF trend. They will end up different, of course, as you master different jobs on different characters, but their abilities have no bearing on their in-game personalities and lore, it's up to you to decide whether Faris is going to be a Summoner who gets to reunite with her dragon in the end or not.
FFVI did a lot of new, intriguing things, but just as many feelsbadman failures. I do like the balance between customization and bespoke mechanics: every character brings something unique to the table (if not always useful) even as they all learn magic and get to use summons. This gives players control without compromising character identities, which I feel is important and worth going out of your way to enable. And the customization is truly unprecedented in FF series, with everyone being able to rise their stats as they wish and learn any magic they fancy in thematic batches, allowing for clever combos and exploits. On the other hand, espers only rising your stats if they're equipped on level up, combined with the otherwise good autoleveling mechanic, leads to a truly bizarre meta where the most optimal way to play involves avoiding leveling up as much as possible until you get the good espers, then overleveling some characters at the expense of others so they wouldn't lose out on stats when rejoining the party. None of it truly matter in the end since optimization is deeply unnecessary to complete the game when Ultima exists, but it sure feels bad to realize how much you've missed if you went at it blind, souring otherwise very ambitious and interesting mechanics. Also, unlike FFV, FFVI throws new toys at you until the very end, but like FFV you need to invest time to make use of them, which leads to things like Meltdown falling by the wayside. In addition, for all that espers are super-important to the narrative (or half of it, anyway), summons are very undercooked and often forgotten, especially compared to excellent realization in FFV.
Finally, FFVII. It continues the interchangeable characters trend, unfortunately, with everyone being defined by the materia they have equipped that could be changed at a whim. While different characters do have different limit breaks, which even matters, especially in the endgame, I feel it's not enough to make them feel truly unique, especially compared to FFVI Blitz or Trance or Rage mechanics. While FFVI sometimes struggled to make different mechanics truly unique (Blitz and Tools are Magic by another name, basically), FFVII could have used its smaller cast to better differentiate the characters from one another, and it's kind of a shame they didn't go farther than just limit breaks. Taken on their own, though, materia are a great concept. As industrialized magic, it fits the setting, its attachment to equipment allows for interesting trade-offs, and the linkage system allows for a truly staggering amounts of combos where every materia not only gives you access to the skills contained therein, but also serves several different secondary functions like granting immunities, additional effects, serving as your counter, etc. It's just a very neat concept that was realized fairly well. The downside, though, is very much like in FFVI (though with less feelsbadman): the game throws toys at you until the very last moment, but demands investment of time to make use of them, leading to potentially great materia languishing in your backpack forever. Their biggest flaw, however, is the total absence from the narrative. Espers were important (at least in early-mid game), getting new ones was often a tragic event where you witnessed an esper die. Jobs in FFV were memories of past heroes empowering you aginst the coming darkness. FFIV skills were a reflection of characters wielding them (and, in the case of Cecil, actively changing with character development). Materia are... nothing. They're theoretically a part of the setting. They're referenced by characters, sometimes. But, aside from big plot-important items that bear little resemblance to what you actually slot into your sword, they don't exist. I feel this is connected to the trend of interchangeable characters: the mechanics are divorced from the world, giving player more tools and control at a cost of immersion.
Well, whatever else can be said about FF series, it was always characterized by ambition. It's not content to recreate a working formula, it always strives to try something new, something bigger, if not always better. For all their flaws and failures, for all that I personally disagree with many choices those games make, the mechanics of each game are always interesting and worth talking about, and I, for one, can't wait for the next entries to surprise us once again.