Some of the most sought after areas are:
Los Santos in the Cerro de la Muerte
Palo Verde National Park in Guanacaste
Tortuguero on the Caribbean and
Corcovado in Osa Peninsula.
(found in the little section in http://AMCostaRica.com where they translate a paragraph or two from one of the local Spanish papers. This one had originally appeared/been translated from an article that appeared in http://LaNacion.com