It depends on what you consider "good looper"
Okita and Karl are pretty much identical in refund but Okita has 20% higher base damage (sum of 70% buffs as opposed to Karl, who gets the Q debuff after damage so 50%), so it's a matter of hitting traits where Karl outdamages Okita.
Karl also has his Heroic King gimmic which can make up for the 20%, but it's not reliable.
Related, I recently made a list of Q loopers that can refund 100% from 0 starting NP, might be able to find it, but from the top of my head, servants that could do it were: Charlemange, SA Okita, Daikokuten and Dantes.
Overall tho, based on my experience, due to how 90++ are structured, aka 1-2 enemies per wave, refund is a really small factor, so, like other things, it's all about dealing enough damage, at which point, Q/B/A is relative to your supports and who has the right mods; luckily, we have a lot of neutral supports now so borrowing the missing piece can work.
For example, here is a meme run of me just throwing all but the kitchen sink onto my Ibuki lol, I only had one Castoria
https://www.youtube.com/watch?v=-ueG7TBryPc