Kamenko's recommendation is pretty good, but depending on your level, a Portuguese dictionary may not be what you need. What's the point in having a dictionary if you don't understand the words involved in the explanation of a concept?
Since it's far from to to make assumptions of your Portuguese level, I vouche for Kamenko's recommendation, though it lacks something important - contextual examples. I've never seen a printed dictionary which offered these, though, so unless taking notes is that important, online dictionaries like priberam are a better option.