Jaffar Theorycraft

First off, I'm extremely jealous that you have a Jaffar, and I wish I could kidnap him from you. :<

For the question, I'd probably go with Deathly Dagger, or at least try out a different one, like Smoke Dagger (for debuffing all stats) or Rogue Dagger (for buffing allies as well) to enhance his role of support and debuffer.

It's just... Deathly Dagger is so amazing at chip damage.
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If you want an honest to god answer
Bench him until a firesweep dagger comes out
Right now they're extremely underbalanced in our meta, though pechatz and Plate have there niches, the rest dont really, unless they're in emblem teams (Linde and Kagero)
If you're dead set Deathly Dagger with refine is your best option, but the possibility of essentially wasting the resources due to if we ever get a better dagger

It's not an easy thing to solve, because there are many different factors coming into it (like the rest of his team and available buffs). Also, I have only a +2 Jaffar throughout multiple pity breaks, so my opinion will not be as educated as it probably should be.

All the dagger options are viable -

Cleaner has a devastating advantage against blade tome mages and all the different (hone/fortify) emblem teams in arena and even outside of emblem teams, it gets enough value if there is just a simple dual/rally skill present or a tactics buff on the enemy team. The problem comes when the enemy depends purely on drives/goads and such or even on debuffs only, which kills its viability, making it very unreliable.

Barb shuriken is in the middle ground here, as it just provides a simple effect, which will always be useful (like proccing moonbow/glimmer on every desperation encounter). As good as it is, it's not really that exciting or even worthy of being associated with Jaffar.

Deathly dagger on the other hand, especially with its effect upgrade gives you a clear advantage and (in my opinion) brings out the most out of Jaffar as possible, because afterall he's a dagger unit and his role is to provide support to his team when dealing with strong enemies. Due to his low attack, he has problems killing some units, which is where Deathly dagger will serve as a finishing tool or as an initiating tool thanks to the brutal chip damage. The magic sweep effect can also allow you to choose a different B slot for him, which can be immensely valuable - like swordbreaker, because he will not be able to double the +10 fast sword in arena, or even chill speed in order to provide further debuffing support for your team...even skills like wings of mercy are viable, because he can jump in and surprise the enemy. This is why I would choose Deathly dagger alongside its special refine over the other dagger options.

Honestly, Deathly Dagger is Jaffar's saving grace in my opinion.
Sothe and Legault are the best for pure offence.

With deathly dagger (eff) Jaffar can savely kill off mages and destroy teams. It's a support heavy tactic, but at least he's got a niche there.
Although with that merge he does have decent defences, you could give him close counter with quick reposte and a close def or smoke seal to make him a pretty crazy bait 'n kill unit.

ps; My Jaffar is spd+ res- (unmerged) and he does his job well, even if I mostly use Legault as my main dagger now.

I love Jaffar and his Deathly Dagger eff/Moonbow/Close Counter/Poison Strike 3/double Savage Blow 3 is AWESOME

Just like everyone else said, his atk is too low for other daggers, his legendary weapon tweaks that so he has a specific niche as a blood drainer basically. And that effect against magic users... yesss

Wow, a +10 Jaffar! Congrats man, I'd love to see him in grand conquests or anywhere, someday. (If he's against me >_____< but I'll still be pleased)

I don't know what kinds of opponents you're up against, whether they'll double you if you only have 40 spd, but if 40 spd is enough to dodge the double, swift sparrow probably increases his survivability. Fury is maybe the safest option because it still grants +3 spd on both phases. (Plus if you decide on Deathly Dagger +eff and find a bunch of mages to pick on, you'll eventually activate desperation anyways)

All of the weapons you suggested there are good. I find the Cleaner extremely situational and actually quite dangerous because you're initiating onto a buffed opponent; I've never recommended it to anyone and I don't see a reason to do it here.

If you're going with Deathly Dagger, I think I'd take the effect upgrade unless you're afraid of people's vantage or desperation. 7 damage usually won't trigger it, but 10 will do it if they don't start with 41 HP, which is very common on squishy mages. If you need more doubles, hone him.

Barb shuriken is a very reasonable try for extra damage because moonbow will fire anytime you double, even in desperation. For trying to one-round an opponent, it's better than Deathly Dagger unless your opponent's defense is less than, like, 5. I really mean 5. However, if you're not trying to one-round your opponent, the 10 chip damage means that your moonbow only does more damage (and to only one opponent) when their def is super high, like, around 50.

Rogue dagger I think is only if you invest heavily in his def (which means double-smoke will have to give way to distant def or something) or if you have a healer or are trying to buff teammates, and this dagger is probably more applicable in Grand Conquests. I'm saying this mainly from my experience with an unmerged Sothe. Jaffar and Sothe aren't especially durable units, so you really need to go hard on their defenses if you want them to take more than 2 hits.