I like Fuddruckers. Should try Five Guys with that in mind. Do they douse the patty with MSG or something? Also, if you can't taste MSG, most likely you can't taste umami.
My favorite is Tornado Burger. I'm not usually huge on greasy foods, but Tornado Burger is pretty much soaked in grease, and I suck it right out of the bun. Mmmm....