Chile, long ago. Very difficult accent, spoken fast, with the mouth shut and word endings casually discarded. Bolivian is slower, Colombian and Venezuelan are fast too, but beautifully enunciated, so even a moderate speaker can understand quite easily. Mexican is a drawl, to me, and Argentinian sounds like Italian!

Chilean telenovelas in the 90s were very good. Mad, surreal comedies, not the weepy, Brazilian melodramas. If you ever get a chance to see "Oro Verde" or "Tic Tac" I recommend them highly ('Tic Tac''s soundtrack was entirely Queen songs by tribute bands, basically a homage to the music and spirit of Freddie Mercury). They have their own Wikipedia pages even!