Old Post Office, downtown St. Louis. Photo taken from my parking spot in the 7th Street Garage. The OPO was recently renovated and now houses part of Webster University and the Court of Appeals*, amongst other bit-nesses. My favorite place inside is the "express" library. The staff is super nice and they always have a good selection of fiction, cookbooks and kids' literature.
When I fist started at my firm the OPO housed the Social Security Office. Nightmare.
*Of note, I have a 100% win rate with the Court of Appeals. disclaimer- I've only argued in the COTA twice.