I can not just pick one European port as a favorite.
Venice Italy is a must see, Rick Steves said in one video I saw that 3 days is enough in Venice. I spent a week there are could have spent another week. Venice has it all, the food, art work, the buildings and the relaxing environment of no cars with water buss and water taxi.
Rome Italy, the city has history, art and food. I have spent a few weeks in Rome over 3 visits and I'd be happy to go back. There is so much to see and eat in Rome.
Greek Island cruise has to be one of the most beautiful cruises to take. The Islands are a must see. Beautiful building, views, great food, wine and beer and nice people. I took 100's of photo's on the islands.
Barcelona Spain was a great port visit, we did the Gaudy excursion since I enjoy architecture, the excursion was well worth the time and money. After the tour we spent the afternoon having lunch and wandering La Rambla visiting galleries, shopping and people watching.
The Azores islands are beautiful to see. We did the island tour up to the volcano for the lakes that ended with a Local Wine and Cheese tasting at a hotel with ocean view. Nice way to spend a day.
I have been to other European ports in Italy, Spain and France but those were my favorites.
My thought on it is if your taking all the time to go to Europe 1-2 day flight in and 1 day home I like to have 2 week, With 2 full days travel time and expense of the air travel I like to have time there to really see and enjoy Europe.