I agree with Jim S-W comment about being inspired by a prototype, it happened to me after reading a 1985 US magazine and generally becoming obsessed by it. The odd thing is tho, although i've been inspired by certain American layouts (particuarly Peter North's), I've never seen a rateable Interurban layout in the flesh that I could say inspired me to have a go.
Anyway, I couldn't list a top three so narrowed my selection down to specific modelling periods I've been interested in:
Teenage GWR branchline phase:
Breedon, Ashburton, and a funny sounding Welsh plan that was in RM in the 1980's and someone will have the name to withing the hour.
Trying something different, Narrow Gauge:
Charlie Insley's Welsh 009, Gordon Gravett's first O-16.5 layout.
Coming of age dabble in N/2mm:
Nether Stowey, Rhosteigne, and generally anything small in 2mm.
Bit more disposable income and modern image:
Boldon Jn, Carstairs (certainly inspired me to build quite a bit of stock for it).
Current American phase:
Any Peter North layout, Rock Springs, and a nice D&RGW layout with superb presentation and sound effects.