I can add a bit to this debate as I spend a large part of the last year living and working in the USA with regular commutes back to blighty.
Agreed Delta is the best of the terrible selection of US airlines. Avoid Atlanta and Washington Dulles like the plague. My experience is that Detroit and Boston are too bad as hubs and I've only once had a problem in Chicago O'Hare and that's over many, many connecting flights over 8/9 years.
Allow plenty of time for idiotic conversations, bag searching, demeaning undressing etc. with the TSA and Immigration. Prepare to be insulted and if someone is actually pleasant to you try not to be too shocked.
Contrary to what others say, the US airlines have a mix of new and old aircraft alike. The problem is that they sweat their assets hard so the wear and tear takes it toll and they don't maintain the interiors that well.
In my experience taking purely internal routes on US airlines, about 50% of flights are late more than 30 minutes and about 20% late by more than 60 minutes. At virtually all US airports except the local feeder airports expect misery and you won't be disappointed.
My rules:-
1)
try and use a non-US airline if at all possible, even if it means a connection
2)
avoid known bad hubs and try and route yourself through the ones you know aren't too bad
3)
try and identify the few 'least bad' US airlines to use for your connections
4)
use the local feeder airports if possible, as the service is much better and it's often far easier than driving to a big hub and checking in there.
5)
pay for at least premium economy if you can, as you'll be treated much better