Probably javascript, its is versatile enough to be used for large applications as well as quick scripting projects. at the same time the open source community around it is quite vibrant so new libraries and projects are constantly coming up to make development on it easier.

I would choose Python as my one programming language to master. Python is a versatile language that can be used for a variety of tasks, from web development to data analysis. It is also relatively easy to learn, with a simple syntax and comprehensive libraries. Additionally, Python is widely adopted and supported by many companies, making it a great choice for a long-term investment in programming skills. Finally, Python is a powerful language that can be used to create complex applications and solve difficult problems. With its wide range of applications and ease of use, Python is an ideal language to master.