Assuming your HF Hassan is NP5, then she should outdamage him, at least for multiple NPs since his Arts buff is 1 turn.
However, Kiritsugu does have some neat features HF Hassan doesn’t (just like how she has her own unique features) so if the situation requires it you might want to use him instead, those features are targetable taunt, ignore invincible, and his NP decreasing enemy charge and ignoring defense.
Yeah, at NP1 Okada Izou is likely outclassed, but at least you have another chance on the rerun!
Since we’re on the topic of ST Arts Assassins, Serenity trades some offense for more debuffs, notably poison damage over time, NP charge reduction, and if the debuffs land, then NP/Skill Seal on her NP. Her debuff success rate third skill improves the chances if used for her NP, bringing the 40% chance up to 90% chance at max.
So if you don’t have any other notable assassins, then those are your main choices in the silver/gold range. I guess Jing Ke counts, but she’s more about burst damage, so probably not the best pick overall.
In the lower range, Cursed Arm is great for crit teams but lacks an NP upgrade to be a good ST option, Sanson is good against Evil targets, and Sasaki is getting his NP upgrade right now, so he can hit hard, but is fragile. And of course, all three of these could use more stats to be more effective.
So HF Hassan is likely still your best choice for overall effectiveness, but some of the others have situations where they can shine, otherwise just wait for more NP levels/more assassins.