I'm running DC. I think in DC, Atlas, Kato, and Athearn Genisis are out in front. Some of Athearn regular engines are really good. Their GP40, GP38, SW1500 and SW100 run very smooth. I've found that the new Athearn and Atlas rolling stock run very well over almost anything, whereas with all of the Bachman rolling stock I have (which is really only a few cars), I've had to replace all their wheels because of constant derailments. Engines and rolling stock built by either Lifelike or Model Power, I don't even waste my money or time on. However most Proto engines run well, but then a few don't as well, either.
So in the final annalysis , I would have to say that in an affordable engine, Atlas is pretty hard to beat. In the high dollar engines, Kato and Athearn Genisis take the prize. Of course, this has been based on my own experiences throughout the years. You and others may have a different view.