Along with Spanish spoken in Bogot? these are some of my favorites. The argentinian (bonaerense) accent because of the Italian inflection and for the way they pronounce LL and Y. At times a little hard to understand but it sounds so cool. For me, along with the Medellin accent, it is sexiest for female spanish speaker. The others I like for their clarity. Which is why tend to have a problem with some caribbean speakers, too much is sacrificed for speed. It sounds choppy and inelegant. As in English, people who enunciate properly sound better to me.
Worst:
Spain, by far. And all their BS about speaking perfect, formal Spanish like Nebrija intended is so annoying. The lisp is like nails on a chalkboard to me.
Argentina. The LL and Y kill me. It sounds very ugly to me.
Peru. Impossible for me to understand, and not especially nice on the ears.