...is Someburros for me.
Have been dining there since it's inception in the late 1980s.
I think what makes it my favorite is the red sauce, the quality ingredients in each dish, the consistency in flavor and deliciousness of my favorite items, quick and (usually) friendly service, and very clean aesthetics.
Haven't found anything like it here in Southern California.
Granted, we're not talking gourmet or knock-your-socks-off cuisine, but for fast food, I find it to be pretty awesome.
I'm curious to read a list of your favorite Mexican food chain restaurants in the Valley.