Ah, this one again, with a few limiting factors thrown in. Always a good question it seems, though any one answer is always unsatisfactory, or at least unsatisfying!
Today, I think I'll go with an K.J. Miller #100 Junior Engineer, because with this "one" model, I can reconfigure into three different steam engines.
Yeah, that's sort of cheating, I know!!! ;c)
If you were to lift the price ceiling, or if I were to get unbelievably lucky, my answer would have been a Jensen #51.