I think I've been to Whataburger once or twice while traveling, and I can't say I recall it very well. Certainly not enough to offer an opinion on the burger quality.
However, I disagree with the idea that having a more expansive menu necessarily makes one place better than another.
I.e., just like Whataburger, McDonalds has a much more expansive menu than In n Out, but I don't think anyone here (incl utee who made the expansive menu point) is going to claim that a McDonalds burger is better than an In n Out burger.
Quality and variety are orthogonal attributes when evaluating a restaurant. I don't want to eat at a place that has an expansive menu if all they serve is various flavors of sh!t. And I'm not going to avoid eating at a place if they only have serve one item I like but that one thing makes the visit worthwhile for me because it's so good.
Maybe that makes Whataburger a better place for a family of five trying to successfully accommodate disparate tastes, but it's a different discussion than whether a core menu item from one place is better than a similar core menu item from another.