My options are teaching buisness english,teaching toefl prep, teaching english grammar or teaching english to kids.

Which one will have the most open positions? Which one will pay the best? Basically, whats my best option for overseas employment?

I don't know where are you. But if you are in the US, I would think that one or two courses in a one or two of those specialties would be sufficient for a TESOL/TESL certificate. Most of your coursework would apply to any of these ESL/EFL careers. I am unaware of any TESOL/TESL certificates that require a specialty. But I may be wrong. However, if you have an undergraduate degree in TESOL/TEFL and are going on for further education in that area, you might need more specific courses.

I think you'll always have more opportunities with adult education, but I don't think that's where the money is. I also think it's going to depend on where you are going. If, for example, you are going to the middle east, I'd say that TOEFL prep would be a good job to have as many middle east students come to the US for advanced study. This might also be the case for other areas of the world.

Business English might be advantageous if you develop an association with an international business. But there would probably be a limited group of people who would be looking for that. Teaching English to kids would be a good position in the US in the public schools. Teaching older children might be good in public and private schools overseas. But it all depends on where you are going.

Hope this gave you some ideas. I think you need to make some decisions and do some research.