Although I'm a LNER modeller normally I do have a small collection of Rivarossi American outline loco's and without doubt the smoothest running and most powerful loco I own is the Nickel Plate Road 2-8-4 it's also quite awsome looking, but it's not British so I'm going to say the new Hornby A1 "Great Northern" is my pick. The A1 in original form is a really elegant loco and it's a superb runner but 8 coaches is about its lot on the local club layout.