The Rose-breasted Grosbeak (in my opinion) is one of the prettiest birds around. It has a wonderful song and when it's in Spring plumage is just a striking bird. Well . . . you can judge for yourself! :)
Rose-breasted Grosbeak:


PS I recommend clicking on the photos to see the larger versions over at my flickr site.
Here's a previous "Learning About Birds" post on Rose-breasted Grosbeaks.