My longest waits have always been at JFK.
Arrived on BA115 on Thursday...we were an hour late arriving and had the misfortune to be behind a Cathay Pacific flight. We were in J but still had to queue for around half an hour, maybe a bit longer.
I wouldn't have wanted to be at the rear of the plane...we could see the queue go right up the 'ramp' and beyond.
Best time at JFK was about 15 mins which is quite acceptable , worst was just over 2 hours.
I think being a smallish terminal it doesn't have the same immigration desk/staffing levels as maybe terminal 8.

I don't know if I've been lucky at ORD but I never seem to wait that long, especially now they have the machines.

I have no experience at the other airports.....have you considered BA1 ? You can pre-clear in Shannon and arrive as a domestic.