Okay, let me get this clear. There is no such thing like best character in the game. I mean it depends a lot on the player. Individually, you can have a best DPS for you, but in general for all players, no such thing exists.
It depends on the playstyle of the character, the various team comps you can build around them and how well the team synchronizes as a whole. And obviously how well you can play with that character. Ganyu is a one the best DPS out in the game, but due to her bow weapon type, many mobile players find it hard to play her optimally.
In that sense, Eula is not the best DPS, but she can be your best DPS depending how well you build her along with the team she goes with. And that goes for most DPS characters out there. I hope that answers your question 🙂