To keep it reasonable, let's cap the size at a Stewart or smaller, and a price of $2500 or less.
Paul
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.
Just one?

That's impossibly hard for addicts!!!!
Ok...I'll play because its only a fantasy
I think I'd be pretty happy looking and running my Marklin 16051 forever.......but then a Jensen 20G or 55G or........................
Going the same route as Daniel, it would be the Märklin convertible given it has 3 configurations and endless options with Metallbaukasten. Close second would be a Jensen 50 because of size and ease of running all day long.
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One of my 7 or 8 convertibles I have collected 😁
https://youtu.be/WAR6s9qYwKo
Jensen 20G Deluxe. It has the power, looks, and just the right size.
Bing 130/116 and complete my collection. its 1 size bigger. seen is 114 and 112 to the right [ Guests cannot view attachments ]
I thing a Jensen 20G would probably be the one for me.
Without a doubt whatsoever...Jensen 20.
Very strong, close to impossible to wear out, and furthermore found within very reasonable price ranges, whether new or used.
Got to gowith the majority and say a Jensen 20G - day in day out it's hard to beat for ease of operation, power and good looks.
Doll 510/2...I really enjoy overtype engines and Doll made some fine examples. [ Guests cannot view attachments ]