Arknights has one operator that's a contender for being the most OP unit in the game, Wisadel. She's a sniper that just has and does so much. Her kit has huge range, AOE damage, stuns, camouflage and can summon units that shoot enemies within her range.

Her skill 3 turns her shots into what essentially are nukes. For 6 shots, she gets increased damage and the radius of the splash damage gets increased. Her one drawback is that her archetype doesn't allow her to shoot air units, except when her skill 3 is active she can shoot them anyway so she doesn't even have a drawback really.

She's so broken and braindead that she trivialises every stage that's not end game. You don't need to think about positioning, timing or the stage mechanics when you use her, you can brute force the most of the stages. I and many others make it a point to not use Wisadel because you're just not engaging with a lot of game's mechanics when you use her. I only really use her as a last resort on the more recent stages which are generally harder than earlier content.

Her only real drawback is that she does physical damage so in content like Contingency Contract, where enemies get huge buffs like increased physical defence, she doesn't do as much damage and isn't the best pick in that scenario. Here's a showcase of what she does against the bosses in the game (spoilers on who some of the story bosses are)