While I cannot answer the OP question of "best seed brand," birds are extremely demanding and require our creativity to provide a healthy diet.
My technique is to provide fresh veggies/fruits, pellet mix, and small quantities of a quality seed, supplemented by various treats. I consider the fresh diet to be best, so 3 to 4 days per week it is the ONLY food available during the day.
During the "off" days they have a mix of Zupreem pellets and seed mix blended by the store. (first ingredient is safflower, no sunflower) Early in the day either a dense whole grain bread or thick portion of oatmeal/oatbran is given, with a small bit of string cheese once a week.