When we visited Germany, we flew into the Koln Bonn Airport (CGN). This is a smaller airport so it…read morewas much less intimidating than Schiphol or Heathrow, which we also flew into.
Immigration lines were short and everything was efficient, although we didn't check bags or fly OUT of this airport, so I can't comment on those aspects. The wifi worked well and the bathrooms were clean and plentiful. I also love the branding/art of the airport with the little car train, suitcase, and plane icons.
The train station is just below the terminal, which is very convenient. We did have to wait a long time for the train, but that's on the train system and not the airport. Thankfully the station is covered from the rain! From the airport, it's a short and fairly cheap ride into Cologne.
If you're coming to Germany, The Koln/Bonn airport is a great option.