I know that Andean people are supposed to speak Spanish more slowly and clearly than coastal people. I've also heard that Loja, Ecuador, is considered to be the best place to learn Spanish in the world because they speak clearly and correctly.
Chile, Argentina, and the Caribbean are supposed to be some of the worst places to learn Spanish because they speak fast and use lots of modifiers and slang. I can tell you that I learned Spanish in rural Chile, and people from other countries have had difficulty understanding me. I've had to work hard to get into the habit of speaking correctly.