This is interesting because I always enjoy the highly detailed layouts, but the ones that seem to grab my attention for longest are not the ones I would have predicted. The last exhibition I attended a couple of years ago had a large 3-rail layout which I could not drag myself away from. There were Castles passing Deltics on passenger trains, and 2-6-4Ts with Co-Bos on goods trains. No worries about correct sleeper spacing, just painted tinplate.
I suppose it brings back the "small boy" in me, and what I would have given in the 1950s to have had access to that layout.
Not strictly an exhibition I know, but my all time favourite is the Gainsborough Model Railway Society's O gauge ECML layout. I could spend all day there. I really dont think I ever grew up.