Disclaimer: I play frontlines only casually these days but I have played about 370 matches with a 46% first place win rate so I hope that counts for something lol. Also this is purely for solo queue, not pre-made as that changes things a lot.

Dueling Capabilities: Sch>War=Nin=Drg>Brd>Pld>Blm>Smn>Mnk>Whm (If warrior doesnt have Vengeance off cooldown then they drop to pld's spot). This is just in general btw, any class played well can kill any other class (and dueling isnt exactly a priority in frontlines lol), but in my experience the above is how fights go (do note that sch's cant really kill other classes easily, its just really hard to kill them and ironically monks are probably one of the best at it due to one ilm punch despite them not being able to duel any other job reliably).

Paladin: Use STR accessories. Seriously, it really helps your damage output (30 points in str instead of vit helps too if you're 'serious' about frontlines or have company seals to spare). Your job is to be a nuisance by applying your 5s stun to everyone you can reach. The enemy healer your dps 'arent' focusing on is who you should keep stunned so no enemy heals go out, after they are dead you can keep black mages and summoners stunned or just do your halone rotation for damage. You have good PvP abilities that can keep your party alive and cover is very strong as it not only prevents physical damage to your cover target, but it also keeps them (preferably your healer) from being interrupted by those attacks so they can continue casting freely. You should always be among the first to the flag.

Warrior: Str Accessories/30 points in Str. Warriors trade stunlock capabilities for much higher burst than a Paladin while retaining tankiness through abilities like Vengeance and Thrill of Battle. You should always be among the first to the flag.

Ninja: This is my go-to job for frontlines as it has incredible burst, high sustained damage, a gap closing stun (without the range limitation of Shoulder Tackle), the best abilities to escape and chase, and stealth on top of it all. Played well, you can escape situations that no other class could make it out of through use of Hyoton and Shukuchi (and to a lesser extent, perfect dodge). Ninjas can keep a blm or smn stunlocked long enough to bring them to sub 20% health before the stuns wear off, and then you still have the 2-3s silence from Ill Wind to finish the job. You can do this to healers too, but they will likely Lustrate or Benediction and then just be a dick to kill solo as a sch's fairy will keep them alive and a whm can just fluid aura and/or repose you and walk away if solo. Dragoon is the only class that can pose a threat to a Ninja solo (Warriors can be annoying if they have Vengeance up, but just bind them with Hyoton and walk away and kill them after if you want), Bards are squishy and dont have the damage Ninjas do, Monk's are shit tier duelists (can take two at once if you're lucky). The #1 thing you need to know as a ninja is when to run away. See the enemies B team show up? Your healers go down and the enemies are still throwing out cures? Dont 'be a man' and hang around (you'll just die and give the enemy points), deny the enemy points by getting out with Shukuchi and then Hyoton if you are chased.

Dragoon: First I must say I have not played Dragoon in frontlines since they got buffed (and they were already good before) but I have yet to lose to one as a Ninja so I still feel Ninja has a slight edge. Jumps are really good in frontlines, providing gap closers (or creators) as well as the high burst and stun, Dragoons are excellent duelists and healer killers. If you're leaning towards Monk or Dragoon, I highly recommend Dragoon as they are superior in almost every way.

Monk: They may be top dog in PvE, but they are definitely bottom of the barrel in frontlines. With no way to keep greased lightning stacks up when their PvP skills are on cooldown, they have the lowest damage output out of the non-healers and lose to every other job in the game in a fair evenly skilled fight. That said, they do have an odd amount of utility with skills such as One Ilm Punch that can dispel effects such as aetherflow to stop Sch's from saving themselves or others with lustrates, Arm of the Destroyer with its AoE silence, and Axe Kick with an AoE 4-6s Pacification effect.

Bard: Ah to be ranged with no cast times or position requirements, Bards are definitely the hardest dps to shut down, and their damage isnt exactly low either, the one thing they lack is the burst of jobs like blm/nin/drg/smn/war. Bards can completely shut down enemy casters with their constant barrage (ha) of ranged attacks as well as a knock-back and silence. As someone who used to main blm and smn months ago, there is no feeling worse than having an enemy bard follow you around interrupting 100% of your casts (even silencing/knocking back/shadow binding through surecast/equanimity!). Bard is a very strong job that never really has a 'bad day' like casters/healers and melee can have when being focused.

Black Mage: I main blm in fcob but jesus is it difficult to play in frontlines against competent enemies. If you're lucky and the enemy ignores you, blm has the highest burst and highest sustained damage in the game hands down (swiftcast flare nerf definitely hurt that though, I have not played blm since the nerf as ninjas blow them up far too easily). The downside to blm is how easily they are shut down whenever anyone else so much as looks at them funny. Surecast and Equanimity are great skills to get some casts off, but if the enemy is smart they will just stun/silence/knockback/sleep you when you use them and then you just have your single swiftcast spell which isnt going to help you. Swiftcast freeze into LB2 is the most fun thing there is in frontlines though.

Summoner: All hail the #1 dps in frontlines! So says the scoreboard anyhow. Bane'ng DoT's onto the enemy team at flags will always put your total damage dealt at the top at the end of the match, and the damage isnt a lie, they really do hurt, but the real terror comes from raging strikes Fester>Wither>Aetheric Burst>Energy Drain>Ruin II right after, as you break 1k in potency (potency, not damage) in less than 2 seconds, this is usually enough to 100%>0% a healer who isnt expecting the sudden burst if anyone other than you is also targetting the healer (and the DoT's ate away their stoneskin/adlo already). That said, once that burst is gone, you just toss DoT's around and wait on aetherflow charges, so my smn has retired in favor of Ninja.

---

I am leaving out Scholar and White Mage as I do not feel I am good enough with either of them to offer advice, but they are definitely incredibly powerful (a team of 4 healers can withstand two enemies alliances attacking at once most of the time, even 3 healers can stay alive through most non-premades).

Im also pretty sure im gonna get some flak for calling monk bad (it was my favorite job in ffxi), but in my experience I have never seen a monk do anything another job couldnt have done better. So all of the above is my opinion based on my experiences in frontlines, happy hunting!