Which graphics engine is best for Mobile?

It depends on exactly what sort of mobile game, but for general 3D games, Unreal or Unity are the go-to popular engines for mobile. Unity probably performs better for low-end with default setup, but Unreal can also be optimized for low-end as well with the correct rendering knowledge. The main benefit for Unreal can be blueprints for rapid gameplay prototyping and better initial setup for maximizing visual graphic fidelity. For 2D games, there are several different options like AppGameKit or GameMaker Studio.