He's a linguist so he'll have a better answer than I do, but basically depending on each country, you'll have a different Spanish, but it's roughly the same for middle / upper class Spanish speakers. In some parts of Central America you get a lot of Indian language + Spanish mixes, some places in the Andes also. The Caribbean varies per island, Puerto Ricans probably having the most slanged, Anglicized and ugly Spanish. Dominicans have a nice sort of... I want to say musical Spanish. Cubans speak kind of like Puerto Ricans, less slanged and far less Anglicisms (unless they're exiles). Some Mexican Spanish can be tough, particularly more rural Indian types.
Chile and Argentina have their own particular dialect, but I find it easy to understand. Then again I'm Puerto Rican, so I don't think they speak Spanish as fucked up as ours anywhere but Mexico. Colombians and Venezuelans, in my experience, have the most widely intelligible Spanish.
In Spain I find their Spanish is mostly understandable. I think Colombians speak my favorite Spanish, if I could make a hierarchy of the best Spanish speakers.