I won’t argue that they’re good pullers, but “some of the best” is probably stretching.
The big heavy articulated steamers really are in a league of their own just because they have a massive(for O scale standards) can motor with a 7-pole skew wound armature and rare earth magnet field. Those are some stout motors-it wouldn’t surprise me if they were the single most expensive component of the locos they were installed in(I’ve seen one former engineer from Right of Way Industries say the same about the motors they used, which were a smaller 8000-series Pittman motor but all ball bearing). Those big motors are geared roughly 20:1(a vertical motor diesel is usually more like 10:1, give or take) and they have about 10lbs of boiler and chassis holding their 12 or 16 driving wheels to the track, and of course generous traction tires around.
For diesels, I’d probably put my Lionel Veranda in the running, although I’ve not done formal pulling power tests with it. It’s a BIG loco with a big diecast shell that weighs probably in the range of 15lbs. Only half the wheels are powered(there are four separate 4-wheel trucks, only two of them are powered) but the power comes from vertical Pittman 8000 series motors, so the motors themselves put out respectable torque figures.
Honorable mention too probably goes to the 90s Alco PAs(another of those Lionel-MTH cooperations), which get a decent amount of added weight thanks to the diecast trucks, have traction tires on 4 wheels, and power comes from two Mabuchi 545 motors. Nearly every other vertical can motored diesel out there uses the much smaller 385 motor, although I’ve not found a typical modern dual can motored diesel to be terribly lacking in power.
I’ll just say too with all of these I’ve mentioned, puling power is really more than I can max them out at with the space I have available. I love putting long strings of coal hoppers behind the big articulateds, and 30 is about what i can manage on my living room floor. I mentioned that brass locos often have issues starting this train(although it gets better as I get more and more converted to Kadees) but diecast has no issues. I can not start this train with a Magnetraction loco, even running on tubular track.