In the smaller locos I think the peak was the South Australian Railways 520 4-8-4 series
Hoping Steamranger can get 520 running again. 621 shouldn't have to carry all the load. (The 620 series is actually the older, smaller model)
Looks a lot like the Pennsylvania T1. The T1 was one of the sharpest looking US steamers, but with four cylinders on only eight axles she slipped too much to be called truly successful.
My personal favorite is the Norfolk and Western J.
Equally at home pulling a long varnish up the side of a mountain or blasting across the open plains. The 611 is still runnable, but the lawyers have it tied up in a roundhouse. Can't get the insurance to run it.