My favorite is a tie between Primanti Brothers sandwich (the infamous one with fries and slaw inside) or the supreme Benkovitz fish sandwich at PNC Park in Pittsburgh. I assume same offerings can be had at Heinz Field for Steelers games, but I have not had opportunity (or money) to experience.
Worst is ANYTHING at Turner Field in Atlanta. Overpriced, very poor quality, extremely limited choice. Only size hot dog offered is huge, bordering on obscene, and about $7….what do you do if your kid wants a hot dog? They don’t want one of these huge things….