Sonic - wish I could find a good one again. They used to be great but all of the ones in my area are just high priced/low quality.
I went to several different In and Out locations in SoCal and still never saw what the big deal was.
My favorite is Cheeburger Cheeburger, a Florida based chain. Burgers are good, shakes are awesome, and the frings ( fries mixed with onion rings ) are to die for. They even sell the fring batter mix in bags. We always bring some home. They're not completely nationwide yet, but getting there. There isn't one in my area. I have to drive a couple of hours for it.