Obviously, saying anything is the "best" is subjective. Everyone will have their favorite locomotive and a reason for choosing it. However, there were some locomotives that stood out from the rest and were maybe just a bit more successful than some others. I am going to mention some of them and why I think so. If you have a different opinion, feel free to send me information on explaining why. Perhaps I'll add them to this page.
Pennsylvania Railroad's K4s Pacific was probably one of the most successful American locomotives of all time. 425 of them were built between 1914 and 1927. Dimensionally, they were not the largest. However, operationally, they were very efficient and were used to haul most all of PRR's passenger trains. An indication of the great respect the Pennsy had for this locomotive is the fact that one of her type was selected as the railroad's monument to its steam power at the famous Horseshoe Curve. That K-4s was later moved to the Railroader's Memorial Museum in Altoona, PA.
Most Hudsons were of roughly the same design. New York Central had the greatest number of Hudsons by far. The class J-1 and J-3a Hudsons of 1927 had 79 inch drivers. They were fast, powerful, very well proportioned, good looking, and may have been the best known steam locomotive.
The class O-8 Mikados of the Great Northern are the easy winners of this type. Built in 1932 by the Great Northern, they were by far the heaviest, fastest, and most powerful of this wheel arrangement. They were arguably the best looking too. Because of their 69 inch drivers and large boilers, their pulling capabilities were in the same league with the NYC&StL Berkshires and some Northerns.
Lima introduced the 2-8-4 wheel arrangement in 1925. Erie improved this design by using slightly larger cylinders and larger, 70-inch drivers making a fast and large 2-8-4. The Nickel Plate (NYC&StL) further improved the design by starting with a C&O T-1, removing one set of drivers, and applying characteristics of the Erie Berkshires. NYC&StL called this class S-1. This same successful design was later used on a number of roads including:
Originally designed for passenger service through the mountains, the Mountain type became a fast dual-service locomotive. The New York Central called them Mohawks. The Mohawks (class L-3) of 1940 had lightweight rods and 69 inch disk drivers. This provided better counter balancing than most and helped the Mohawks to cruise at 80 MPH.
This is a tough one. The reason this category is hard to pick is because for many railroads, the Northern was the ultimate and most modern fast freight and passenger locomotive. Most Northerns were well designed, modern engines. Perhaps a few stand out though:
The B&O "Big Sixes" were an exception to the normal 2-10-2 "drag" locomotives. The "Big Sixes" (class S-1) were powerful and faster than the other 2-10-2s. Built in 1923, they had large 64 inch drivers and were used as fast freight engines. They carried long Vanderbilt tenders. They were called "Big Sixes" because their engine numbers were in the 6000s.
A case could also be made for the 140 locomotives in the Santa Fe 3800 class. These locomotives were instrumental in heavy freight movement during WWII and weren't retired until the mid-50s. Their tractive effort ranked in the top five of this arrangement. Because of their successful career and the number in the class, they could be considered.
The C&O T-1 2-10-4s were built in 1930 by Lima. Up to this time, most 2-10-4s were built as "drag" locomotives with smaller drivers. While helping to provide greater tractive effort, the smaller drivers made it difficult to balance the weight of the side rods and valve gear which was necessary to operate at greater speeds. In designing their T-1 2-10-4s, the C&O basically took an Erie Berkshire and stretched it into a 2-10-4. These T-1s had 69 inch drivers. Unlike most other 2-10-4s, the T-1s were powerful and fast. This basic design was used for most 2-10-4s built afterward.
One exception to this were the Santa Fe 2-10-4s. They were built with 74 inch drivers. The C&O 2-10-4s were heavier and could exert more tractive effort. However, the Santa Fe 2-10-4s were faster. No other 2-10-4s were built with drivers this large.
Another class of 2-10-4s worth mentioning were the T-1-b class of 1938 used by the Canadian Pacific. The ten locomotives of this class were semi-streamlined and used for passenger service through the Rockies. The CP called them "Selkirks".
Wait a minute. There wasn't ever a 4-10-4 built in the U.S., was there? Well, there was, sort of. In 1944 the PRR built #6131, a duplex-drive 4-4-6-4. A re-arrangement of the drivers and cylinders had solved the problems of the Q-1. In 1945, 25 more Q-2s were built (6175-6199). The Q-2s were the most powerful (HP) and strongest (tractive effort) of all non-articulating steam locomotives. They were the most successful of all duplex-drive locomotives. However, because of dieselization, most had a short life and were stored by 1949. The Q-2s represented the ultimate in steam freight development in America. In a way, they could be considered the 4-10-4s of America that were never built. Sadly, none were saved.
The best compound articulated has be the N&W Y6 series (2-8-8-2). These engines were simply the best of the breed, capable of high tractive effort and yet able to pull at up to 50 mph. They could be run in either the efficient "compound" mode as well as in the "stump-pulling" simple-expansion mode where they could generate an astonishing 166,000 pounds of pulling force.