Ok, actually writing down my thoughts on the carrier list. This proved a lot less straightforward than I thought it would mostly because primary skillsets are (correctly) often factored in when considering how good of a carrier a class is.
1. Wizard is obvious
2. Ninja is obvious
For 3 and 4 I went back and forth between various options (including Priest and Geomancer). I settled on Oracle and Time Mage for a few reasons. They are very solid magic carriers (in a game where magic carriers are a lot more important than physical carriers) and they offer significantly better skillsets than black magic (or Geomancy, for that matter). I'm...not sure Oracle over Time Mage is correct, though.
Oracle makes a decent carrier cause their stats and equipment are a little better than Time Mage's overall (better equipment, worse stats, but...overall better). And they can refill their own MP with Spell Absorb, cast Pray Faith on themselves if they're carrying a faith based skillset, use Life Drain in the many situations where Life Drain is good. They are probably better at doing supportive actions towards Summon Magic, for example (Pray Faith when that's good, Spell Absorb when out of MP, Life Drain against Zodiac bosses so Summon doesn't need to bother with Lich).
Time Mage makes a decent carrier cause their primary is better than Oracle's Primary. Even without Short Charge, if you're a Time Mage with Draw Out, for example, and you fail your teleport you can just...haste yourself, which as a draw out user you probably care about more than Oracle's tricks. With Short Charge they can use Meteor. There's an argument that Summoner/Time Mage is better than Summoner/Oracle, not because Time Mage does anything really to support the Summoner skillset (summoners don't want any of Time Mage's buff spells), but because of Meteor. There's also Time Mage/Chemist, which is an excellent support character while not giving up damage (whereas an Oracle/Chemist would be giving up damage). Calculator...probably likes Oracle more than Time Mage, but doesn't like either one very much.
Hmm...should probably reverse the order of these two; more of the skillsets that can be carried seem to lean Time Mage.
3. Time Mage
4. Oracle
5. Chemist ...I mean, this is a bit of a hard one to place. But...guns and Throw Item. Just thinking back to co-op games I've played and how often Chemist just comes up. Don't really have a hard logical argument for the exact placement of this one other than "I feel like I see Chemist carrier kinda often". Although...honestly, I've seen Chemist carrier more often than Oracle carrier if I'm being completely honest. But on the flip side, Chemist carrier also usually a stepping stone and not the final destination, just pretty good for a while.
Next clump is a bit of a mess.
Priest is a good carrier for Calculator, cause a lot of times the thing you care about as calc is speed. But it's also a miserable carrier for most other MA based skillsets (Summon, Draw Out, Black Magic). It's fine for Yin Yang I guess, YYM doesn't care about the low MA, does like the speed. Priest as a carrier has situational uses for Time Magic if you're trying to deal with one specific story fight where you need to speed match with a fast enemy to drop a meteor on them. Also worth noting Priest has PA, can honestly get away with being a carrier for Jump since okayish PA multiplier, high speed, able to wear clothes, nice primary (revival--if your Lancer wants revival it's not like they're going to Chemist--terrible PA or Monk--terrible revival). I guess the primary revival means I'm probably underselling it as a Samurai carrier too. (Again, not like your Samurai wants to go to Monk or Chemist for revival). And also primary revival honestly makes this at least a little bit interesting for special characters like Agrias with Equip Sword.
Geo is an okay carrier for everything; it can carry physical or magical stuff, and offers a shield and 4 move and an acceptable primary in exchange for notably lower stats than dedicated magic/dedicated physical carriers like Wizard/Ninja. Also has sword, can be a carrier for Agrias.
Summon offers a really good primary, slightly slower speed (which can actually be good for speed matching slow enemies--primarily enemy summoners). Most of the time the slower speed just hurts so much though. So it can be a situational carrier like Priest where you pull it out for specific fights for speed manipulation. Most of the dual class combinations that would be interested in Summoner as one of the two skillsets would not pick Summoner as the carrier, however...with a few notable exceptions. Summoner/Chemist picks Summoner; that's probably the best argument. Summoner/Samurai picks Summoner--kind-of a weird dual class cause they do similar things, and both want a ton of JP. Summoner/Calc picks Summoner (but Summoner/Calc is a really weird combo). Summoner/Bard picks Summoner (even weirder).
Howeverrrrrrrrrrrrrr.... The way speed works, though, there's also just windows when Summoner is really, really good, because they've reached a speed point where they match Wizard. (Five levels between 13-17 when they are speed 6 and other classes haven't hit speed 7, four levels between 31-34 when they are speed 7 and other classes haven't hit speed 8. Two levels between 50-51 where they are speed 8 and other classes haven't hit speed 9). During these small level windows they're...better than Oracle, better than Time Mage, honestly basically Wizard tier for a few levels (still worse MA than Wizard, but way better primary).
Mmm...yeah, ok, I'm probably underselling Summoner.
It still belongs in the same clump as Geomancer/Priest. These are mostly situational picks you'll bust out for a fight or series of and usually not an evergreen pick. But like...probably towards the top of the trio? On Priest/Geo...yeah Priest over Geo still looks ok to me. Want speed/want usable revival with the primary without bad stats probably beats the Geo callings of want shields/want 4 move/want an above average physical attack. I might be underselling how good Geo is as a carrier for Draw Out in particular, though (where the 4 move and shields make it arguably an Evergreen option for that one skillset). Might be worth thinking about this some more...for now:
6. Summoner
7. Priest
8. Geomancer
Alright...
9. Monk on the one hand, unlike the last three which have brief windows of shining as a carrier class, this is an evergreen carrier class...but only for exactly one skillset (Punch Art), with I guess some arguments that you can carry Jump as a Monk with Equip Spear...maybe? Just...yeah, definitely lower than Geo for sure.
OK, Knight and Archer probably should be thought about. Much like the Geo/Priest/Summoner trio they're situational, something you pull out for a fight or two, except just...worse and useful as a carrier on worse builds. Knight is solid in Chapter 1, a fine carrier for Agrias before you get Geo or after you get Knight Swords since it has better stats than her base class, and on rare occasions a go-to for generic characters if the Excalibur is not in use. Archer is nice in some specific story fights where you know you can take the high ground and have ridiculous range with longbows. Mmm...yeah the Knight stuff feels like it matters more.
10. Knight
11. Lancer At the point when I put Lancer 13th I was basically grasping at straws. But thinking on it some more Lancers have Spears, so if you haven't learned Equip Spear yet, you use them as a carrier for Jump! Lancer/Dancer combos sometimes go for the high HP cause dance doesn't need speed. Lancer/Chemist might ride out Lancer until Thief Hats really forced their hand. And of course Lancer does just have stellar equipment in Chapter 2. Much like Monk, Lancer is a carrier for its own skillset, except unlike Monk this is a temporary thing that ends half way through Chapter 3 and Chapter 4 really kicks you out of the class hard. Hmm...still, 13th is probably wrong. The more I think about it, it's got at least one maybe one and a half chapters like Monk where it's the best carrier for its own skillset. Probably bumps it up above Archer and Mediator.
12. Mediator is there for using black robes with Elemental guns, and also for using innate Monster Talk. Mmm...honestly, there probably is an argument that Mediator should be ahead of Archer, actually. Switching for Longbows in specific fights is an option...but it's an option you take if you want Archer JP anyway and you're just timing when you'll get the most relevance out of the class. But Monster Talk tends to force you into Mediator whether you want more Mediator JP or not.
13. Archer
14. Dancer You can probably think of lots of dual-class setups that would use Dancer as one of the two classes. But how many can you think of that would actually choose to use Dancer as the carrier instead of the secondary? There's...Agrias with Equip Sword. If you want Dance/Agrias, you probably wind up in Dancer. There's Dance/Jump, where long-term you probably opt for the clothes job just to get Thief Hat and Power Sleeve. Actually, you know what? That's enough to bump this up over Thief. Definitely not enough to bump it up over Archer or Mediator--I've seen both Archer and Mediator carrier come up in real games, I haven't seen carrier Dancer. But I'm starting to believe it's not that outlandish.
15. Thief If you don't have Ninja unlocked, this has 4 move, 4 jump and speed. Also maybe you want to steal the Genji Equipment, and your secondary is something weird. Also, maybe you want to steal heart a key target in a story battle where you already know the gender. IDK, these are all stretches but it still sounds more impactful than Mime.
16. Mime Finally, a list where Mime is not last! Yeah, I mean, the stats are pathetic, but I can think of actual reasons to bring it when all skills are learned, I mean...it's still a sketchy inclusion, but it does do something different.
17. Samurai The best I can come up for Samurai if you already have the skills you want is various weird dual class setups. Chemist+Samurai if you really want Phoenix Down (but that's iffy you should use Draw Out Priest instead, it's better). Dancer+Samurai if you really want both and really want high MA (this is a really hard sell, cause Dancer's MA mult is higher than Samurai's, but they can't equip robes, so I guess if you want Wizard Robe then maybe?) Lancer+Samurai--Samurai has higher PA and MA using both skillsets better (if we ignore the low HP and lack of shield). As a short term training setup, Samurai with Jump is legit--Samurais jump real nice. But as a non-training setup? No, doesn't make sense to me. It's...close to me putting them above Mime, but none of these actually sound...good?
18. Squire Maybe you don't have Thief or Geo unlocked and want 4 move. Or maybe you don't have Geo unlocked, don't have Equip Sword learned and you don't have enough damage against Velius so you want some Twist Headband Power Sleeve Squires. Like...for some reason you sent all five of your characters to Archer for the entire game, but you're a newbie enough player to struggle with the Archer SCC on Velius, and now you're looking at Platina Sword and thinking it can dig you out of the hole you dug yourself into. Yeah............ It's way more of a stretch than the Samurai stuff, but I can see a weird scenario where I would advise a real human being to switch to Squire for one fight. Does...this mean Squire should be above Samurai?
19. Bard Charge+20 with Bloody Strings and a Mime?? (I mean, technically you can, but surely that's not actually a serious build). There's maybe some dual-class arguments here. Bard/Samurai, Bard MA is notably higher. I guess do this on Ramza, the obvious candidate for all Bard builds for even more MA. Bard not being able to equip robes and having garbage HP so they don't really want to melee kinda ruins it though. But...at the end of the day, going for a dual-class arguments mean that you care about both skillsets, and are accepting slightly worse stats in exchange for a better skillset. Is Sing that good to make this a real choice over, say, Wizard or Time Mage with Draw Out? I'm thinking no.
20. Calculator There are no dual-class combos with Calculator where, once training is done, you would ever pick Calculator over the other class. There just aren't. Even classes with much less MA that can't equip robes for Chameleon Robes such as Thief...just do status Calc instead.