I read those product reviews faithfully back in the day... the Williams Crown Edition (brass) Big Boy made around 1990 was tested at over four pounds of pull (this equated to 130 mixed-vintage freight cars.) It had a single large Mabuchi motor geared at 48:1 to its sixteen driving wheels. The wheels and axles were sprung in their sub-frames to ensure good contact with the track. Each set of driving wheels had four rubber tires for a total of eight tires. I'm not sure how much it actually weighed, but there was a substantial lead weight in the boiler, which brought the total weight in line with today's die-cast engines.
Because of the weight, the rubber tires, spring suspension , and a super-low gear ratio, my money would still be on the Williams Big Boy. Also, the drive line (U-joints, gearboxes, etc.) was robust enough to deliver that power to the wheels.
With a model, weight is the most important consideration for drawbar pull, then motor power and gear ratio. I personally despise rubber tires. But a loco with tires is going to outpull anything without them, regardless of track or grade.
If you're talking Postwar, then I would say one of the later F3's, like a 2378, or perhaps a GG1. An FM TrainMaster would be close. 773 Hudson for steam, but it probably pulls less than the diesels. I know that around 1980, ten Lionel GG1s pulled a 750-car train to set what I think was a world record at the time. That's 75 cars each, not to shabby!
In "real life" an AC6000 would be a formidable opponent to something like a Y-6b. That would be an amazing contest! I love steam, but the AC6000 can do a much better job controlling the train on the downhill side ;-)
