I have found that the best brand and quality of boys underwear are those sold by Disney.  Long lasting quality that will wear great through many many washes.  I am in the process of potty training and have been through many pairs.  Many designs are available but my favorites are Hanny Manny, Mickey Mouse Clubhouse, and The Incredibles.  Some other good brands that are a close second are Hanes and Kid Connection (by Wal-Mart).  Sesame Street offers styles in Big Bird, Elmo, and others.  Kid Connection offers some cute designs are firetrucks, footballs, and stars.