Unreal Engine

You could call this the most impressive visual scripting system in existence, and the reason people pressured Unity to come out with their own native VSL. Blueprints did it first and are respected by a bunch of industry professionals for a variety of reasons. Its speed, feature set, and stability are all excellent.

You get a complete gameplay scripting system designed around a node-based visual interface that eliminates (or greatly reduces) the need for coding. Blueprints lets you use many concepts traditionally available only to programmers, and you can implement these concepts in a much easier way since there is no code or syntax.

You can modify literally any gameplay element, change game rules, arrange weapon drops and item pickups, changes in the state of the game world, procedurally generated levels, etc. Everything from simple prototypes to complex AAA game development is possible within Blueprints. To showcase the capability of Blueprints, devs at EPIC games created a flappy bird clone within just 1 day.

At its core, Blueprint is an event-driven system. When an event takes place (user presses X key or a certain program executes the desired result), functions tied to those events will be run on actors within the game. You can use Blueprints to create customizable prefabs, playable game characters, HUDs, etc.

There is a ton of built-in documentation and tutorial videos on how to use Blueprints. It is a very scalable and customizable platform and can be adapter to even the most complex of AA or AAA games.