what is the most markertable course in the world?

Since we are now in the computer generation, i would say Course related to computers is in demand. Ex., computer engg, science, IT. I wish i took one of those ive mentioned.:)

I'd argue that an English teacher is the most marketable person in the world. Admittedly, you might not earn a great deal (if you're working in developing countries, for example), but as long as English remains an international language, there will be a need and demand to learn it, so anyone who can teach it can effectively look worldwide for their next job. It's also worth considering just how many different "styles" of teaching you could potentially do, ranging from class teaching and private tutoring to "Business English" courses for professionals, or academic English for students who wish to eventually attend an English-medium university.