Today at Discovery News you can find out why dolphins are now believed to be the world's second most intelligent animals, with only humans displaying greater brainpower.
Intelligence itself is a very loaded issue. It's difficult to compare one individual's brilliance with that of another within the same species, much less to attempt to compare intelligence among multiple species. Intelligence is just one component of a species' survival, so one can argue that spiders have evolved to be as smart as they need to be for their species to continue, rats are as brainy as they need to be, and so on.