Favorite is Chicago thin crust pizza (have heard it called tavern style as well) - that’s really the true Chicago pizza (that’s what all the old neighborhood bars and pizza places serve in many parts of the city).
Vito and Nick’s, as mentioned above, it the best-known example, but lots of other local ones. Flo Santos is a good, newer, well-known option.
Thin, crisp, cooked in a big gas pizza oven (usually).