Mechanical Engineering. Firstly because I am already studying it, but it’s also a super broad field, so if I’m an expert in all of it, I’ve just become an expert in like a dozen different fields.
Aerospace, biomedical, materials science, robotics, mechatronics, fluid dynamics, dynamic systems & controls, nuclear energy and all the other energy production types like fusion, even some branches in quantum computing and weird physics like Alcubierre drives.
Sure maybe it’s limited to the extent of where those fields are now, but I basically just became an expert in all of human technology.
Guillotines could cut the economy in half too; I think we may be ready for that