I studied Spanish in Spain (not from scratch). General consensus seems to be that anywhere in South America/Latin America is good for learning Spanish with the obvious exceptions of places where it is not the official language (Brazil etc.) Some areas have stronger local accents but that shouldn't put you off entirely- just be aware that Chilean, Caribbean Colombian, Dominican Spanish etc. are quite noticeably different from other varieties with different vocabulary- and Argentine and Uruguayan Spanish have slightly different grammar in some ways. But again, that shouldn't put you off.
I found in Guatemala (where Spanish is very much a second language for many indigenous people), many people couldn't speak Spanish completely fluently, the result being that they spoke quite slowly. In general I found Guatemalan Spanish very easy to understand.
Certain places (Antigua Guatemala, Cusco etc) are hotspots for learning Spanish due to cheap and generally good tuition- but they are also full of gringos so getting an immersive experience will be much harder.