Cost Rica is a great country. Very clean, very safe (outside of San Jose) with magnificent scenery, live volcanoes, rain forests, surfing beaches, huge range of flora and fauna, friendly locals and you can drink the tap water. It's an undiscovered paradise and I can't wait to go back. Beware of flying through Miami though it's a nightmare. Better to go through Madrid.
Peru is interesting but dangerous and dirty, Bolivia is cleaner but not as interesting. Equador mainland I enjoyed but after Costa Rica I was disappointed with Galapagos and everyone (but me) was very sick on Galapagos. Argentina has great landscape and culture and is more sophisticated but the jewel in the crown in Chile and especially Torres del Paine national park. We took some horses out and it was the best ride of my life. For Argentina do not fly Aerolineas Argentina as they are the worst airline I have ever flown with, they left me in Madrid for 3 days with no prospect of ever leaving and I had to take a circuitous route via New York to escape.