Well, I don't have the hundreds of flights under my belt that some denizens here do, but of the maybe around 100 flights I'd estimate I've taken from the UK to the west coast over the last 20 years or so, for pleasantness/lack of hassle I'd mark the carriers that I've flown with as follows:
Air New Zealand 9/10
Virgin 8/10
BA 7/10
United 5/10
American 5/10
Delta 4/10