I'd pick Katsuyu, but only if i have byakugo to boost her. When i think about it Katsuyu is superior to her summon peers both in defense and offense !
Defense wise, she whitstanded CST without taking a scratch so to put her down for good you need an attack at least stronger than CST and there aren't many in the no god tier category of fighters.
Offensively, her corrosive acid melts through rock with ease so i assume it is as dangerous as that of Mei which barely melts Susano ribcage; now imagine if this acid is boosted with Byakugo like tsunade did to Onoki jinton and covers a large scale too ? It'd destroy most of defenses.
Katsuyu can also divide herself into small entities and fire acid from different angles which makes it even harder for the enemy to dodge.
If someone wanted to use her dividing ability creatively he can make her cover the whole battlefield with her clones and use reverse summon to the location the user want himself to be in, like how Minato FTG works.
- Against Manda : He have no way to harm her and if he tries to eat her, she can kill him from the inside with acid.
- Against Bunta : Even Kn6 Naruto corrosive chakra didn't burn her so Gamayu endan isn't working either, she will eventually outlast him or kill him with acid (noted to kill even someone as resilient as Orochimaru)
- Against Enma : He can't harm her either.
So yeah, IMO Katsuyu is the strongest summon.
