The strongest weapons and armor are found in chests in the final dungeon (Evangel castle). Note that Thoma can’t change his weapon, and it gets stronger as you progress the story.
Lise: Vampire Sword, and perfect armor.
Leticia: Vampire Rod, and principle dress
Mylene: Vampire knife, and killer maid dress.
Thoma: Tracing blood blade, and guardian robe.
Best equipment (Knights of Grayfang)
I let Lise, Leticia and Mylene use the "Treasure Room" weapons throughout the game. These weapons boost the damage inflicted on the first three turns by 50%, which is more than enough to make up for the better stats of other weapons. In terms of (physical) defense, the Colosseum Armor that can be obtained at the Arena exchange is simply outstanding, and there are many more armor pieces with great effects that more than compensate for lower defense values.
There are also strong armors as rewards from the secret arena’s second and final stages. I am a little disappointed that the vampire weapons don’t have a different model. This game is somewhat more generous with the gatcha rewards as I got the highest tier weapons using the premium tickets you get from the arena and the achievement awards.