The best thing to get at Peters' is the milkshakes, hands-down. The burgers, fries, onion rings... I can pass on. Combining and testing the different milkshake flavours is quite fun, if you're into that kind of thing.
I think that Peters' Drive-In is 'famous' in Calgary because there isn't anything else quite like it in the area; it's an experience. It isn't the best, but hey, it's Peters'.