Over on Model Airplane Maker, there is a beautiful post recollecting an old friend and hobby store owner who offered that everyone has one great model in them. It’s a well-written post with lots of photos of beautiful models; you should read it then come back here.
In a model railroading context, the idea of one great model is interesting. Will it be a piece of rolling stock or a building or other scenic element. What happens when that superstar appears on the layout? Does it shine brightly to diminish all the other models that make up the whole? Should you stop striving lest it does?
Then there is a question of greatness as it pertains to our hobby. There is so much to focus on in model railroading. If we think about rolling stock, for example, a great model must not only take top marks for looks, but also needs to function. Is a building without interior lighting great enough to be your one great model?
I don’t know. I do know that I have made some nice models, and the idea that one day I might make a great model fills me with hope.
Pembroke:87 
Model railways is different to most other modelling disciplines, which tend to be about individual models and dioramas. We generally focus on building layouts, with controlled motion representing the everyday operations of a real railway, in a real or imagined place. Rather than applying this concept to the individual models on the layout, why not apply it to the layout itself?
That could make sense in the UK where loads of people seem to complete multiple layouts. Here in North America, most people never finish a single layout! Could an unfinished model be considered great?
I think that most people over here never finish layouts, too. I think I have started over a dozen, but only one was ever close to completion. Even there, there were a couple of things in need of addition before I would have said, “Enough! It’s done.”