Every field is the same, it depends about what you really like. If you like it, you will be more curious and want to work harder. And if you do a good work, company will notice you. And if you keep up a good work from there you will get an idea how to raise your profit so you can get more money.
The answer changes with time, you would have to see what area in the market is lacking professionals. If the oil business needs Mechanical Engineers then they will pay better, now they are laying-off people to reduce cost. Mechanical engineers with knowledge of electronic equipment can be in a better position for tech world. It follows the rule of offer and demand. Low offer with high demand, the salary goes higher. Do what you like and be the best at it.
All engineering careers can be well paid and satisfying. Like all things, it depends on how much you invest of yourself in earning a good result. Engineering, including Mechanical Engineering, is a hugely diverse field of study and work. Keep an open mind about which field you choose to study and even more about where you might work. Your engineering education will lead you to discover your natural talents and preferences. This is important because when you work in a field where you are happy, success will follow. Be prepared to change direction as you follow your strengths but stick to your plan to work hard and get a good education in your field. Best wishes!