MIA and LHR. Put your walking shoes on, your going tramping around for a while.
Haven't done MIA since the early 90s, but still remember. You think it's bad as a US citizen? Try it as a foreigner. Two hours in immigration was the best I ever managed, by quite some margin. I can recall several occasions where all but one of the desks were for US citizens. We used to pack water and snacks specifically for the immigration queue.
Having said that, this seems to be a general theme at US airports. It's almost like they don't want us there...
Arriving into LHR T3, with the inevitable death march towards immigration, followed by a long, long walk down into the bowels of the building, where you arrive at the queue for the T5 bus, which is three buses deep - that's always a personal favourite. Since I'm going to have to re-clear security in T5 anyway, one day I'm going to try going landside and hopping on a service bus, see if it's any more relaxing that way.
However, both MIA and LHR pale into insignificance beside the fustercluck that is CDG. Connected there twice, HEL-CDG-LYS and back, so all-Schengen. Most of it is a grey concrete blur, thankfully, but I remember getting off the plane and walking forever, turning several needlessly tight corners, and finding ourselves pushing along a grey concrete walkway beside a kid with a giant pirate sword. How the hell did he get that through sec- No. Oh, you have got to be shitting me. Really? We're landside? Really. We're landside. And now someone is begging off us. Wallet and passport into the backpack, and clutch it tight. And still plenty more dingy darkness before hitting the gigantic queue to get back through security. Still a whole hell of a lot more walking (actually, near running by this point) before ending up in a bus gate with a busted vending machine and all the charm of a truck-stop toilet. Mercifully we didn't have to spend more than a minute or so of our two-hour connection there. And going by others' stories of CDG, I think we had it good.
Didn't even have time to visit the Air Scotland lounge.
By way of contrast, I found that GVA just worked, even with His Popiness arriving as we were trying to leave. Gutted that we couldn't get out onto the roof terrace outside the lounge, but on the whole, GVA's actually a fairly pleasant place to be. We did need to ask Information where to find the bus to Grenoble, which could've been better signed.
HEL's easy enough to arrive into or depart from, but the 30-minute connection times are somewhat optimistic. Definitely doable if it's quiet and you don't get a bus transfer to completely the wrong end of the airport, but neither can be counted on. I've even seen it where the OUL flight was docked at the furthest non-Schengen gate, then we all got thrown out the back door and bussed all the way back to the farthest reaches of the Schengen side. Worked for me, I was staying in HEL and ended up right by the railway station, but those who were going to LHR and had to run the entire length of the airport to get back to that same aircraft (all in 25 minutes) will have been less than impressed. There are perfectly usable bus gates right under immigration - I know this because, when arriving into HEL and expecting the bus to drop us by the station, an entire A320ful of domestic passengers ended up having to clear immigration. Good times... But if you've got time, and it isn't rammed full because it's July, HEL works. There are some quite pleasant spaces to be, too.
Let him that hath understanding count the number of the beast:
For it is the number of a man; and its number is One hundred threescore and twelve.