My experience flying in Europe is limited.
We really liked Amsterdam. It's relatively small, very well organized, clean, has good eating options and is easy to reach or leave by train. We liked Manchester for the same reasons. LHR and Gatwick aren't bad, but they aren't great either. Barcelona is good and easy. Madrid is a little more complex in terms of getting to the trains, but not bad.
I've only transfered in Paris, but it was a disorganized mob scene at boarder control, and the busing from terminal to terminal was a mess. I gather it's expensive to get to city center as well. I landed in Rome only once over 30 years ago, so my negative opinion of it might not be fair.
In the U.S. and Canada, I like a lot of little and medium sized airports: Albuquerque and Portland being the ones I use most often. But we like Vancouver and Salt Lake too. Denver is great in the airport, but not so great for ground transportation. I actually like O'hare. It's big, but its well signed, there's plenty of food, and you can take the metro to the city center. San Francisco isn't bad. Neither is NY.
Flying through Los Vegas is good. But the car rental bus system leaves a lot to be desired, such as not waiting in 100 weather outside.
I don't like DFW. It seems needlessly spread out, and the large people transport vehicles are a nuisance. In fairness, I always seem to have delayed flights there which colors my feelings...
I don't mind flying into or out of Atlanta, but I hate transferring there. The train system is nuisance.
Seattle is good, unless you are running from a commuter flight to a main terminal, and that's usually what I do there.
I don't like the DC airports. Dirty, complicated, and lacking in food choices.
But most of all, I hate Los Angeles. The food choices are often horrid. Getting from one terminal to another is a mess, and often involves standing outside in the heat. It's poorly signed. And, not thier fault, but I'm usually dressed for much colder climes.