MUC would be my first choice since the showers rock in the Senators lounge. Much easier to transit than FRA, though its not rocket science.

I should note that FRA departures on US are practically in another time zone.... not very friendly and no departure lounge nearby. To me any connection on US via FRA would be a deal breaker.

Avoid US on the non 330's....

If you can find a way on Swiss I hear good things... can never get miles to work out of BOS