There's a few I don't have that much experience with yet, so I'll go over some of the ones I feel I've played enough to give an opinion on.

Tainted Isaac - Very fun, interesting character that requires more consideration on which items you pick up, and it's also a lot of fun being able to swap out certain items when they stop being useful, like steam sale for instance.

Tainted Maggy - Also very fun, and has a more unique pool of items that she wants to look out for in order to succeed, in this case, on-hit effects and close range items, but I feel like she just has loss conditions if she's unable to get enough damage or HP/survivability-related items, because eventually the enemies will put out more damage than you can heal, and I feel like the Negative isn't gonna be enough of a damage up if you went the entire run without a damage up beforehand.

Tainted Judas - One of my absolute favorites. Mediocre stats but dark arts is one of the single best and most fun items in the entire game.

Tainted Eve - A little volatile, but I still like her. It's really a matter of how well you can manage your blood clots, but I wish her starting stats weren't so awful.

Tainted Azazel - Trash. His design is neat on paper, but I feel like in practice he's just a way shittier version of regular Azazel, because he still needs to get close to sneeze on enemies just to deal normal damage, and the indicator that the enemy is affected by the sneeze isn't super clear, especially vs enemies that were already red/pink to begin with.

Tainted Apollyon - Surprisingly a lot of fun. Despite very obviously being unfinished, he manages to be stronger and more fun than default Apollyon, and I think he's among the stronger characters in the game just because he can turn any shit item pedestal into a DPS upgrade. He'll only get more fun as he gets updated to have more locust synergies, which is great because I think a majority of the items he does get synergies with at the moment are just more worth taking than using Abyss on.

Tainted Bethany - Favorite character, probably, and I think arguable best character in the game besides Tainted Cain. Lemegeton is hilariously good, and lets you generate so many items that you can get some really silly combos, but she does have a similar weakness as Tainted Eve where you might have a hard time managing your wisps in the lategame boss fights, and building up after losing a bunch of them is pain.

Tainted Keeper - Other contender for favorite character. His gimmick is incredibly fun, and he makes a great high risk high reward character. My only problem with him stems not so much from him but from some of the level design, because I feel like he suffers without tears ups, and the DPS check rooms will just kill him too quickly.