London, Bristol and South Wales by John Jay. We never got to see Paddington or South Wales, but Reading (including Sonning Cutting) and Bristol massively impressed me. It was a huge layout.
Vivien Thompson's Eastbourne, which seemed to keep growing. What both these had in common with Buckingham, Jenkinson's eventual Little Long Drag and many others already mentioned was that they were multi-station layouts, systems really. Railways that went places in a very real sense.
Also loved D. H. Neale's amazing garden railway with superb concrete viaduct. John Charman's Charford, small but perfectly formed.
Not sure how they would measure up now, but I loved them as a kid.