Whataburger is my favorite place if I'm only counting large chains with drive through windows and other fast food restaurant features.
The food is almost always fresh, plenty of customization options like grilled onions and jalapeños. Almost everything on the menu is good quality for fast food. Being open 24 hours is a nice bonus.
If small regional chains count Burger Street is the best, really good burgers, great curly fries and delicious frozen lemonade.
Braums ain't what it used to be. It's really hit or miss these days.