Based on personal experience:

Goods (highest-lowest)
=================
Costa Rica*
Argentina (Long flight)
Brazil (No spanish of course)
Lima (Machu Pichu a must!)
Santiago (long flight)
Panama*
Mexico
Dominican Republic (assuming Caribean is ok)

*Spanish is easier to understand in Panama and Costa Rica (pronunciation).

Avoids
======
Venezuela
Colombia

If you're looking for a real sleeper of a place to kick back, scuba, snorkel or just chill - Bonaire (sm Island part of Neth Anitlles - no spanish but they speak papimiento - a combo of spanish/dutch/french - wild!)

Edit: Visas to all/most should be doable if you already have your passport - here is a link to visa requirements.