Which Dec Emulation is the MOST useful and Versatile?

[Fred Cisin]

On Sun, 29 Oct 2017, Dave Wade via cctalk wrote:
> I am not sure they invented computer emulation. I think that the concept
> Emulation/Simulation is as old as, or perhaps even older than computing.

Henry Ford is attributed with "car racing began 5 minutes after the second 
car was built."
BUT, I'm not sure whether the second computer caared about emulating the 
first one.