The Pit in North Scott. It use to be a swimming pool turned into the gym.
Honestly, As an Iowan this is hard to say or admit. If you want to see stellar gyms you need to venture into the Illinois side of the Quad Cities. The best when packed is tough to say, but Rock Island Fieldhouse and Wharton Fieldhouse(Moline) are the best 2 gyms IMO. I would say East Moline United Township is just behind. Also, Rock Island Alleman isn't great at basketball but that place when filled is fairly impressive also.
Davenport Central has a solid gym. Clinton(The problem is the floor isn't good last I knew) and Maquoketa have decent gyms also.
Though the gym doesn't have wooden seats, which I think makes gyms much better. Assumption has a nice gym with the track around it(Similar setup to Chariton, but more seating) and 2 big levels for a smaller school. It is impressive when full also and when they host wrestling it is neat to see under the lights.