>>3829623
The best one early on is the cow, you can actually get her right after the tutorial too. The dragon is also quite good, but gets hard countered by some fights in Ovitri. As soon as you unlock genesis stone skills in the postgame, the Unidentified and mimic are the two best damage dealers in the game.
This was a tier list shared by SQ-L, the anon who makes all those challenge run videos on Youtube. Such as this https://youtu.be/Jj-AfE-t_b0?si=wK77zS5r5iRQf7on
He listed reasonings for all the tier placements here. It's actually a great read.
https://pastebin.com/Nkn7tecE
I think take all these rankings with a grain of salt, I've seen his other videos and he uses Fiede, dragon, scylla and mimic girl for damage dealers. He used mimic girl for the final superboss of the game too. Just be careful of boss spoilers if you do look at his other IR videos though.

The safest damage dealer to max out first is the unidentified, she’s the best damage dealer in the game next to the mimic girl. Maxing out the robot knight is also a great idea because of passive where she covers anyone if she’s over 80% hp. But also bear in mind that you can really use whoever you want, there are multiple strategies available in this game. For example, the lamia and snow woman are very good in Ovitri since lots of the bosses are weak to ice. And in Unelma’s dungeon, you’ll need to make up party comps on the fly because of the gimmick there.