I flew AA ORD-LHR on a 763 about 15 months ago and found the service and legroom(Y Class) to be quite nice. The return trip was nice too.
If you can, take AA's last ORD-LHR flight, which is the one I took. It is a 763, and since it is the last flight out, there aren't as many pax as most have caught earlier flights. This was the case with mine, as it was rainy in Chicago, but I'd give AA 98(or 99, forget which one) a shot.
Give me a break, I created this username when I was a kid...