Interesting to think about:
Never: FRA (only been MUC a few times), AUH, DOH, HEL, TPE
Only Transiting: DXB, VIE, ICN, DFW, ZRH, GVA
Only Arriving: PEK
Only Departing: PVG
Never on FRA is most surprising for me given my travel patterns and prior loyalty to UA/*A for many years. I also keep meaning to actually stop and enjoy Austria but have only transited it instead of stopping there (really enjoy the transit experience there actually - might be why the FRA is a never)