S scale 1:64 - its the perfect size and delightfully idiosyncratic! Cambrian, light railways, industrial, narrow gauge and American shortlines. Also 1:152 2mm Finescale and 2mmNG, and some overseas modelling in 1:160 N scale.
I don't mind whether a loco is red, black or green, or if it goes 'chuff chuff' or 'brumm brumm'. If it has flanged wheels and runs on rails, then I'm interested!!