Blender's built in game engine actually! Its been my engine of choice since I first learned how to code. However, I decided its about time I graduated to a REAL game engine. And I chose unity as my next choice since a lot of the stuff I learned in blender could easily be carried over.
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Awesome! Take a look at Godot Engine. It's completely free (open-source) and no royalty cut if you succeed. It also deals with some nuances that Unity has (moderately advanced stuff). Offers 4 languages (visual script, gdscript, C# and C++). Visual script is like blueprints in unreal. gdscript is their own dynamic language (much like python + javascript kinda thing), C# - well, no need to explain this and C++ - they call it GDNative. It's their own system that allows you to create custom nodes (base entity in godot) in C++ without recompiling the entire engine .