Best/ viable Assassin?

I don't think people use tank Jakob/Matthew as their attack stat won't do much even if they can tank, although True Damage Jaffar is pretty good as a softener for units with high defences, such as Zephiel (if he doesn't have Distant Counter u can get free damage) and Effie as well since leaving them open to attack during enemy phase is terrifying. Then a mage or another unit can secure the kill after Jaffar attacks.

Yes it is true that Kagero is the best Assassin out of all of them but I believe that Jaffar is very close. Jaffar with his native weapon can be very strong. Even without Savage blow or Poison Strike, he can pretty much guarantee any kill for the next attacker after he has attacked someone. This is because he lowers the Def/Res by 7 and deals 7 true damage afterwards. That is basically like dealing 14 damage after the battle is over. This works really well with Brave weapon users because the -7 Def/Res can translate into 14 damage because they attack twice. I think Jaffar is really good with his vanilla skills and could be better improved with Savage Blow and Poison Strike.

Kagero gets the bonus of being colorless, allowing her to hit all colors equally, with a weakness only to a few tomes, as well as having better bulk than mages. I don't feel like swapping her one is really equal. I tend to run mine with Seal Defense so she can support the team against non-infantry units. Her attack is high enough to kill a lot of cavalry and fliers, even without the bonus damage.

The only two assassins I think are worth even looking at are Kagero and Jaffar. The others, despite tankiness, can't deal damage whatsoever.
Anyway, for Kagero and Jaffar, aside from both being dagger users, they serve very different purposes. Jaffar is definitely more situational - his main purpose, with his dagger, is to chip away (especially with the fact that poison strike stacks with his weapon) and debuff tanks for another unit to come in and finish them in one turn. Kagero is a glass cannon, low defense and high offense (esp. with the bonus on infantry). Both of them are more than viable, it just depends on what roles they need to fill in a team.