Just three? Well, today's three are:
Buckingham Great Central - Peter Denny. A complete system, with all aspects of the layout realised to a consistent standard: design, scenery, buildings, timetable, signalling: all in a credible geographical and historical "aspic" (Iain Rice's word).
The Little Long Drag - David Jenkinson. Again, a system, not just one scene. A shame it was never completed.
Ravensbourne = G Iliffe Stokes (and Mrs Stokes!). When I first saw the photographs of the harbour scene from this in a 1960s RM I was unsure whether it was a model or a real scene. The best buildings, trees and scenic effects I had seen at the time, and beautifully photographed. Only the Triang 0-6-0's crude wheels gave the game away.
But I cannot swear that tomorrow's might be different. For instance Charlie Wehrli's Milwher and Llenli (sp?) cast a completely different light on what a model railway might be.
There may be more recent layouts which are technically better, and RMWeb has its share of these, but the three I have listed were inspiring to me when at an impressionable age, and therefore have had the most lasting effect on my modeling aims (if not my achievements).