Of the four I'd spend the lions share in Cologne. A little bit of everything. The whole city was razed during WWII, except for the Cathedral. Beautiful church.
If you're into cars (which you should be according to your employ) then definitely hit Stuttgart for the museums. I'm not much of a...
Kaywoodie / Watch City Cigar Fat Bastard
The Cavendish in these WCC English-esk blends sweetens the room without interfering with the English taste.
Thank you, Andy (@ak2000).