To me, Da Vinci takes top slot mainly for self-sufficiency. A huge amount of personal batteries over her skills makes her very simple to use out of the box, even if her ceiling doesn’t get much higher. Healing off Natural Body also makes her a great fit for Black Grail, even on long-ish fights where it starts to matter.
Anastasia is a close second by default because of our knowledge of Castoria and how easily she can loop in that setup, with bonus points for being quite capable in the meantime if you have enough looping support. Even if she does fail to kill the 3rd wave, her stun normally reduces the size of the problem, both stopping the enemies actions and their animations, so the time blowout isn’t that huge either. Plus, she is just very fun, both in terms of playstyle and character.
Caster Nero is good when there’s something sticky to get rid of, like a high HP ghost, and she’s capable of NPing twice in a fight without too much help. If you’ve got the resources, she plays very nicely in with Merlin and Helena - a superscope on Nero and anything with 60%+ on Helena (I normally use my MLB DB but Sakurascope does the job just fine), combined with 10% from the Mystic Code of your choosing means 3 NPs, with a good mix of buffs, for 3 waves of dead Assassins, easy. But she does need the most propping up for that magical 3-turn clear.
In terms of a one-stop-shop, though, I do have to agree with those mentioning Murasaki. She can farm at the very least adequately, and the extra anti-boss tools in her kit, the debuff immune/buff wipe immune combo especially, make her a more capable all-rounder on a limited budget, and her anti-Demonic buff can be ridiculously useful for offing irritating midbosses.