I have had In-N-Out.  I like Whataburger better.
But then again, I got to eat from the first Whataburger opened and that pretty much set my preferences in motion.
My second favorite was the A&W drive ins.  Frosted glass mugs full of fresh A&W root beer.  And those burgers.  I dunno.  Maybe I am waxing nostalgic, but that was some good stuff.