Save for Ishtar Or Musashi or Scathach

For sure its musashi. Her skills cover all her weaknesses and they are consistent while offering substantial steroids to herself even without outside help from buffs. She basically has no problems with how overloaded her kit is. The buffs from stuff like waver and merlin are considered overkill on her

Meanwhile Ishtar runs on chance based Invulnerability. And her mana burst though substantial is linked to a one turn delay. She does have the advantage of offering more support because musashi is very selfish with her abilities. She loses out in a pure dps and full burst setup to musashi. She is a rin face though so I will roll for her. Also her NP hits like a truck with all the buffs stacked on her but not without support.

Shishou is unfortunately is stuck with the worst card type in the game atm. She also cant use both of her skills without her NP. Which leads to another problem in that if she cant crit her NP gain is really bad. So it takes a very long time for her to put out damage without support. In other words she is very team reliant.

So yeah overall Musashi is the best on her own or with a team backing her up. Close 2nd with a team is Ishtar as she is an excellent servant. Down in the dregs is lovely shishou stuck with the quick is dead meme. At least until she ascends to become the quick messiah with her Skadi Caster form