C++, but the Visual Scripting makes it an extremely easy and powerful learning point which let's you not need to learn written C++ for a long while!
Though obviously, it really depends on what you want to make and for which systems, some engines are better designed for a specific kind of games than others, though it doesn't mean it's not possible to still do it. UE4 specifically is best for 3D games for PC and consoles rather than mobile/web titles and 2D games.