I have been teaching English in South America for the past few years and really enjoy working with the students, living abroad, and almost every aspect of the ESL life. My dream is to teach at a foreign university, but I dont know if I should go for a Masters in ESL or Adult Education. Or would I need a PhD, and if so, in what field? Thanks in advance.

If your desire is to teach ENGLISH in foreign countries, then by all means getting an ESL degree is your best bet. If you just want to teach abroad then Adult Education may be your best bet, but more research may be required in regards to the teaching requirments of individual countries.