I fly yearly or more to many of these airports.
LHR is down to what you are flying. If you are flying BA, you land and take off in T5 and it will be quick and painless, leaving you time for a nice meal at Plane Food. In one instance, I turned left out of the aircraft, went through empty security (as the only transferring passenger) and had a quick chat with the immigration officer about whether Plane Food was open yet (no :( ), and literally 3 minutes after deplaning, I was next to my gate. Make that 20 minutes if you have to go back to A gates (or whatever the ones with the restaurants are). Last time I got bored and decided to walk between A and C gates, which was a fun if eery experience down an empty tunnel several km long; sort of beats the crowded gate train.
If you are flying not-BA, then, you're not LHR's priority and it might suck, especially with a terminal change. I haven't done this in years for precisely that reason, but I remember cramped terminals, impossibly complicated options to get between them, and generally bad service throughout. Somehow there are always works at LHR, just like with the tube in London, where you check not to see which lines are down but which are up today. Last Christmas, the inter-terminal transfers were done via a free shuttle bus that used the motorway...
I hear you on the FRA walks. But Lufthansa reliably flies A380s internationally, even if the food sucks. And that's a guaranteed quiet flight vs the 50% chance of a crummy old 747 on BA... still, it must be at least 1-2km! And this time you have no time for the much better food options, and a lot of people are walking with you.
ZRH is reliably halfway between those two - the big plane and small plane buildings are separated by a medium length walkway which is always the same. However food is so damn expensive!
I would fly AF if I could just for the better food (and free flow champagne, even if it's bottom shelf) and guaranteed A380 again. But every single of my CDG experiences has indeed been a horror story. The place is like a study on how to screw up an airport. In one case, the only airside toilet was blocked for cleaning, which wasn't ongoing because the cleaning lady felt like taking an extended break, reading her paper and insulting those like me enquiring about whether we could possibly use the toilet before immigration queues...
For Americans living in New York or Washington, I'd recommend having a look at GVA. It's a small, very efficient airport with a lot of connections all over Europe and the Middle East (including cheap ones as an EasyJet hub) and has decent lounges. I think both United and Delta fly direct from these cities. Plus, if your layover is long, you can always get out and take the (free) 6 minute train to the city.