I was thinking about asking this same question!
Probably 1871-1883ish. I really like variety of passenger cars, freight cars, and locomotives during that period. Such an odd mix of goodies. Equally important, the lettering, line work, and colors on the rolling stock were just plain gorgeous.
By the late 1880s, the really small power was out of service/sold off, as were the 4-wheel passenger cars and such. During this period, I consider the diamond stacked locomotives and freight cars to take a distant backseat to the other narrow gauge operations...while they were equal early on. In particular, I dislike the headlights with visors.
Sometime after the turn of the century, when those hideous and unsuccessful Vauclain compounds (with cool tenders) showed up on the seen, it starts to get better. It still isn't the early years for me, but it rivals or exceeds pre-flying Grande era which would be my next favorite period. Saddletank 2-8-0s are pretty cool...
Michael