As far as fast-food or restaurant burgers are concerned, the best I've ever had are from Yellow Basket Restaurants. There are only 3 locations, all in southern California (Santa Ana, Menifee, and Temecula), so I can only get them when I'm on vacation out west. If any of you west coasters ever get a chance, check 'em out. They're absolutely delicious.
Red Robin burgers are awesome, too!