Hi abdurresitcan,
USA
There is no one best subject in electronics, and don't forget the specialist areas: reliability, electromagnetic comparability, safety, quality assurance engineer, technical author, technical sales, systems engineer... These areas are always in demand and can be very lucrative, especially if you start your own company and become a consultant in a specialist area.
In addition to hardware engineering, there is software, and this is the area with the biggest demand, and arguably the highest remuneration.
Don't forget, to progress up the ladder, you will have to move into management, either technical, project, or general, so your interpersonal skills, presentation skills, and writing skills should develop along with your technical skills as you gain more experience.

Just to give you an idea of what happens. You have designed and developed the most fantastic piece of equipment and you are in the lab putting the finishing touches to your creation, and the project manager strolls in and says the customer is visiting next week, can you do a 20 minute presentation on your design, so it is best to be prepared.