Played on Hard mode. Still haven't finished, but still fairly confident (super-late C7).
Freelancer: 3/10. Okay, the good: is an authentically real option on Caldisla & early C1 thanks to Ise-no-Kami, Examine & Divining Rod are useful and quick. Later on, Poison Immunity is strangely one of the best immunities (giving up an accessory to block status is painful, Victoria / Qada / Ominas exist) and I guess JP up is okay for grinding (if you already dunked 10 JLs into Freelancer). For very lategame, you can do degenerate crap with Mimic & BP-costing abilities if that's your ball of wax (it isn't mine) and can get good base stats if you grind an absolute ton (but if you grind a lot you don't need the base stats since your level will be high?). The bad: the stats & damage post Ancheim means you should never ever use this unless grinding up for those useful skills. Sigh, would be 1.5/10 if not for the skills.
White Mage: 9/10. Utterly essential. MP a bit of an issue, and they're quite constrained in Armor choice (Agnes sucked at WM for awhile, and I eventually figured out it was the Star Corset or whatever that Praline dropped not having the Mind boost), but MT healing is invaluable, and they occasionally hit air/holy weakness for okay damage. Angelic Ward is pretty rad too. Finally, while MDef doesn't come up as often as you'd think in BD, there do exist magic-heavy enemies (V&V!) and hey look great MDef + MDef skills if it's a mage boss or something.
Monk: 4/10 Very erratic. Falls off hard in C1, then gets badass staves in C3 + Acrobatics to Crescent Moon everything with, then falls off again. HP+10% is cheap and randomly handy especially vs. Qada/Profiteur, and P. Atk +10% is another cheap slot-filler. Still… not really a fan of 'em post C4.
Black Mage: 7/10 ? For C0-C4: Most enemies have an elemental weakness, and a decent number of bosses do too (crystal guardians, Heinkel, Barbarossa, Victor, Braev), and those that don't have a weakness can be given one by Withering Ripple. So a good start. However, MP becomes a serious problem fast once the L3 & L5 -aras and -agas hit, especially if you're blowing the enemies up on turn 1 with quadra actions. Very dependent on either encounter-adjustment to no-encounters on your boss run, or else having a Salve-Maker pal to top up their MP. Also, L5/L6 Black Magic is depressingly late - it wasn't until near the end of C4 I had L5. HOWEVER. If you want an attack mage, levelling BM is basically required, as it features the mighty Pierce M. Defense skill. This skill single-handedly changes C5-C8 mages from "does the same or worse damage than fighters at the cost of MP" to having excellent damage again. That said, Black Magic does kind of pale compared to Time Magic late, but does have the virtue of being ST for the team-fights and having Aspir to reload MP latefight without need for a Salve-Maker or blowing precious Elixirs.
Knight: 8/10. Very solid earlygame thanks to MPless 4x Stomp & good defense, picks up Two Hands which is great. Falls off a bit for awhile. Then gets the Two Shields ability, which is insanely busted and awesome with Chivalry or Holy Arts. The P. Def +10/30 up skills are quite useful as well when making your ubertank who shrugs off all physical-typed damage, which is a lot.
Thief: 5/10. Very fast, can Life Thief like a pro with a bow, and does more damage than Monk anyway in C1/C2. Also powers up Salve-Maker with steals if you care (I don't). Godspeed Strike is more a DL thing, in-game you can be setting up buffs & debuffs such that vanilla damage does better, but is okay if you don't have that for some reason. Anyway, falls off lategame where not equipping a shield = quick ticket to the afterlife, and Life Thief is less reliable than Utsusemi for light-armor not-dying purposes.
Merchant: 2/10. More Money grinding and that's it. I guess if you refuse to turn off encounters in C5-C8 you'll have enough spare cash to actually use Commerce in C8?
Time Mage: 8/10. Black Mage trading elemental variety for more damage. I don't really care about Haste/Slow, but Gaia Gear Quaga wrecks dudes in C3, and is respectable in C4/C5. Late C5+ they get the mighty Meteor which is an easy 9999 damage with Pierce M. Def stolen from BM, notably better than Dark (multihits & Pierce M. Def synergize well). Meteor doesn't even require equipping Gaia Gear, too. Has some cheap status for randoms if you care like BM (Stop) and Time Slip was nice for scarier random ambushes in C2-C3. Warning: If the off-class Pierce M. Def didn't exist, lategame TM usability would collapse, so make of that what you will.
Spell Fencer: 6/10. Pretty great for C1-C4 - MP-efficient random busting, hits all the boss elemental weakness, etc. No particularly compelling passive skills, though, and falls off a bit in C4-C8 where it's competing with the likes of Pirate & Vampire.
Ranger: 6/10. Hits All The Weaknesses, decent speed, Targeting for bosses that use Default. Lategame gets super physicals off that skill. Lategame bow use denies a Shield which is a tad YOLO though.
Summoner: 4/10? Very hard to rate. Summoner made tough randoms who were extremely likely to generate wipes for my underleveled team in C3-C4 doable, a feat not accomplishable by any other class at the time. Flip side, if you do the mass-Summoning Brave surge for 3 battles you're out of MP. And if you turn off encounters to save MP, why do you care about the MT-focus of Summoner anyway? Anyway, they have the best & safest skillset for random-grinding, especially near an inn, so there's always that.
Valkyrie: 4/10. Crescent Moon is good. Everything else is averageish at best. If you're fighting through a ton of C3 randoms and then facing a boss they're a weaker Summoner that drains MP less, I suppose.
Red Mage: 5/10? They're pretty good in C3 if you train 'em up, and they're acceptable in C4 and beyond as carriers for another magic skillset off their good speed & defensive stats - MT Cura healing is sorta okay for awhile, Dispel is randomly handy, and then set Time Magic or Black Magic for real offense. Skills pretty neat too, the "get BP when statused" is quite helpful & cheap for some bosses. That said, you'll eventually hit the point where their skillset is almost worthless outside of Dispel & Raise, which is unfortunate.
Performer: 7/10. Solid MT buffs. Especially before Pierce M. Def arrives and physical damage is the best in late C3 + C4, Love Power seriously speeds up bosses, and I've Got Your Back has its place too. MP is a bit of an issue when Singing is used by non-Performers, and C7+ fights put you under so much pressure that it can be hard to get the turns to use the buffs if you slap Singing on your healer, but it's still doable.
Pirate: 8/10. Amazing, obsoletes many other fighty classes. Defang & Skull Bash are both fantastic, P. Def & Speed debuffing have their place too, and axes + Axe Lore are great, especially if you grab a Fox Tail. Even good for random-grinding-near-inn thanks to Double Damage.
Salve Maker: 5/10. I don't care about the class at all or any of the passives but Medication is a decent skill to set as a secondary strictly for Compounding. I only know two mixes: Quarter Elixir & Resurrection, but those are two pretty good ones to have - they let you unload MP with impunity, and Arise from WM is insanely-MP expensive, so sometimes having ST full-resurrection MPless is handy. If you're hardcore enough to fight the boss after actually fighting randoms, Medication pretty handy for in-dungeon MP restoration, too.
Swordmaster: 3/10. You can set up goofy counter-strats with the aggro-raising accessory, but if we're going down that road, I'd rather use Ninja's Transience instead, honestly. The Ninja can do relevant other things like Default if they aren't targeted and Utsusemi hangs around, while the SM is forced to spam their reactions. The SM strategy gets real chancy in the C7 onward teamfights, too, where there's tons of mixed damage from multiple targets meaning that the odds of just wasting your turn and dying soar. Super lategame, you can do some dumb things with the Freelancer/Swordmaster Prayer/Squeaky Wheel/Mimic/Slow World combo but lalala don't care.
Saving grace is using katanas, which have probably the best ultimate in the game in +150% Speed as long as the music plays. I never bothered with TM/Performer speed buffs because they don't last long enough and fall off on death, but katana ults can potentially last a nice long time with chaining and don't care about death. Oh and do a bunch of damage for free too, that's nice. Of course DK uses Katanas too...
Ninja: 6/10. Utsusemi potentially cheeses some boss fights and covers for the atrocious durability, and dual-Assassin Dagger attacks is funny vs. randoms for awhile, and the speed is obviously good. Still, not overly impressed with the class itself, even if Ninjutsu is a solid secondary to slap on someone else. EDIT: And Kairai is sadly good, if boring.
Arcanist: 3/10. All the BP-related abilities are too "fair" and affect both sides. All the BM-improving passives are worse than Pierce M. Def which already eats 3 slots, although Save BM MP is good for the other 2 slots. Oh and their durability is comparable to wet cardboard. Sacrifice is a good buff at least, and Absorb M. Damage is sometimes useful? Don't really care about Status Amp, if I desperately want to status out bosses I'll use debuffing specials.
Spiritmaster: 8/10. Holy One allows you to get away with Curada for your healer saving a bunch of MP, the stats are pretty decent, and the skillset is quite useful vs. some of the nastiest bosses in the game - Alternis, the 6 Dragons, refights of Victoria, Finale Bugzilla. A silly MP-BP trick if you're running Mimic and/or Compounding, too.
Templar: 7/10. Champion tank like Knight. Giant Slayer quite decent damage against bosses, too (1.5), and Radiant Blast smashes up Defaulting bosses (at great MP cost). Also being the slowest class in the game can weirdly be a benefit, as there are some things you explicitly want to go last, like Phoenix Down revival or Desperation. Later on turns into a Desperation machine, especially with two-shields from Knight. Armor Lore is handy for other classes. EDIT: Oh yeah, Default Guard is completely awesome as well.
Dark Knight: 8.5/10. If you want to do Dark Nebula spam it's greedy in that it requires you to trade-down to Dark Shields, which require stealing to have enough of them early, and then encourages you to lamely have your entire attack force be Dark Knights. And Dark Nebula is kinda late in the skill chain. And it's possible to have hilarious failure after weakening your own team if the initiative goes the wrong way. And despite being great damage, it's still usually worse than 9999 damage Meteors. But really I'm just making excuses for the manifest strengths everyone else already mentioned: DK damage chews through everything with MT and obsoletes the damage-focused physical classes if you want, leaving only the debuff niche of Pirate & Vamp and the tanking niche of Knight & Templar.
Vampire: 7/10. They're a late-gained elemental Pirate variant that chews through MP faster, but I'm not really convinced that they're better than Pirate, at least not without some hard-to-get Genomes (Fireball, White Wind). Even if they were exactly as good as Pirate, the extra work in acquiring the Genomes would count a bit against 'em, and the elemental damage is a bit less amazing lategame. Light Armor use is also unfortunate. That said, if you care, they do mock randoms forever with Battle Thirst, or even Praline's summoned goons.
Note: I don't particularly hold the long sidequest to access Vamps & Conjurers against them, since you're getting plot & new enemies to keep things interesting; it's not like the Veldt where you fight enemies you've already seen for no reward.
Conjurer: 2/10. Ew self-only buffs. They should have risked slightly obsoleting Performer and/or Summoner by either attaching damage to 'em, or else making them MT and MP-expensive. Theoretically has worth as a carrier off good speed & stats, but Spiritmaster & Vampire do the same thing for a mage carrier and fighter carrier respectively, and they have skillsets worth a damn. Evades a 1/10 because if you're grinding in a place not near an inn Conjurer passive is great (and the Fire Temple has among the best gold per encounter in the game), and Steady MP Recovery might have value for long boss fights, and Save SM MP is good if you're running a Summoner. Marginal, but it's something vaguely relevant, and I usually save 1/10 for "totally useless and/or active liability." (EDIT: And damnit it seems teh summons have rare use after all, mostly just Hresvelger.) LVP.