a) IAH, DFW and MSP - I've spent lots of quality time in every other hub city for the major US airlines (present and past), but have managed to never have a connection at these three.
Also, considering how much of my international travel has been on LH, I find it surprising I've never gone through MUC; it's always FRA.
d) Although I've connected many times in ATL, I've never left the airport.