Low star assassin

Fumma generates more stars, and he's AOE. Cursed arm Hassan is ST. Choose based on what you want from them.

Second best stargen after Jack (5*) is Fuuma, he also has better stats for being silver.
But the AOE NP is better at clearing mobs, not suitable for bosses.
And he can give others an evade, that rare as hell (and usefull), whih increases stargen for 3 turns. And can debuff all the enemies on 1st skill.

CA Hassan has lower stats, lower stargen (still good), ST NP (with low chances of instant death, but dont try using on servants...). He has Protection from the Wind (Protection from Arrows with quality stargen up for 3 turns instead of the semi-useless def up), so, he has a lot more of staying power. Can generate few free stars on first skills.

Depends on what you want. Kotarou is better at farming, CA Hassan, even being more weak, is better for harder content.

I would say Kotaro first, since their ability have been explained above.
Good for help you farm everyday and yes outside Scat and Opera he's the best F2P assassin Farmers out there. Shuten is pretty expensive.
Crit star will flow like a river if you insert Hans with Kotarou + Crit DPS.
Allows you to have almost Guaranteed Crit every turn.
Plus Atk down and Def up also that targetable evade will make your team even tougher.
I can even imagine putting Saberlot there, that Crit bypass FGO class system.