The following is my opinion. Others may have their own thoughts.
In my previous life as an "O" gauger and also in my newly restarted world of "O" gauge, I have made the discovery that my best running locomotives are semi-scale Lionel GG1s, with two motors and my 624 type NW-2 switchers. They run smoother than any other locomotives I own with my 2343s coming in a close second.
I run strictly conventional on tubular track with O31 curves and O22 turnouts, using a post-war ZW transformer. My 624 switcher has a Dallee E-Unit that actually enhances slow running.
The above locos have Pullmor type motors. I also have two K-Line semi-scale GG1s that run very smooth, but these have can motors. They also have traction tires which I am not particularly fond of. However, the K-Line traction tires seem to be a better quality than the ones on MTH locos.
My diesels are smoother running than my steam locos. I attribute this to the fact that the diesels have articulated trucks. The rigid frames of the steamers fight a bit through my tight curves. These are slow running observations.
My little 41 and 51 type switchers also run well, even at slow speeds. Although it's kind of like listening to a blender.....LOL