Whataburger. Period. For the fast-food burger, it can't be beat. It's one of the reasons that I have not ever wanted to leave the state of Texas - I recall those '80s commercials for Whataburger featuring Nolan Ryan talking up the Whataburger, and you couldn't get them in places like Philly, NY, Boston ...
There are places around Katy where I live and Sugarland where I work where there are really good, non-fast-food burgers (9ers Grill, Red's Grill in Katy, Lone Star Grill, Cliff's in Sugarland).
I'm not a huge fan of the 5-Guys burger.