Before moving to Asia I was based in the Caribbean and had to transit through MIA most weeks! It was brilliant when they introduced the ESTA machines which negated the need to queue in the immigration line which after 11am in the morning meant a transit of anything less than 3 hours was undoable!
If you had just carry on luggage with the ESTA machines you could be through immigration and customs and through security in about 25 mins which for the US convoluted methods is a dream, however if you were travelling with luggage especially from Jamaica you would not see your luggage for more than 1 hour as they went through each and every piece.
My point, when MIA works it works well, the Customs people are not pleasant but generally the airport and the system (for ESTA) is great, but why oh why do American points of entry not recognise International transiting passengers and ease them on their way, the amount of times I saw people having missed their connections due to the onerous procedures and then they have to land them in the country overnight!
So scaled for our US cousins the worst airports in my opinion for transiting or indeed having any experience is
The best is Las Vegas, they just want you through and spending money second is Boston Logan always helpful and quick to transfer
There are two worst LAX and JFK for 2 busy airports the whole process is so 1950’s