I voted for "transition era" as I'm definitely interested in the historical side of things and really like steam and early diesels. The 1930s & WW2 would be my next favorite time periods.
I also model a lot of British stuff, and the time periods are a little different there because steam ran later into the late-60s. So, regarding the British scene, my favorite periods would be late-50s to mid-60s (which is the British transition era), followed by the late-40s/early-50s and then the 1930s.
I think I could also get interested in the early-20th century railway scene, but it does seem to be a very different kettle of fish (locos being much simpler & smaller, etc.).
Rob