Here in California...In-N-Out reigns supreme...but while I like it, I don't go out of my way for it.
I saw Culvers listed above and when I travel to the midWest, I always stop in...not for a burger but for Frozen Custard...a treat I can't find everywhere.
Personally, I love McDonalds for Breakfast. I eat at Starbucks a lot too....but no one makes a better Egg McMuffin.
I really enjoy Del Taco which I think it primarily a West Coast thing. I also enjoy Chipotle.