As with everything , depends on the model.
All of the current brands models will run happily on any peco/hornby track, better than hornby/bachmann usually as they tend to have better wheel profiles and more uniform back to back tolerences, due to the published NMRA standards.
However, some of the bigger US stuff (particularly the larger steam locos) overhang going around small radius curves is not pleasing, and some need minimum radius' of 22" or more. Similar with the standard hornby points.
As for brands, its all subjective in the main.
Kato and Atlas are, in my opinion, top of the tree for smooth running (not just US outline, but anywhere), Athearn Genesis , Proto 2000 Lifelike , Broadway are pretty much at a level, slightly below the former two, but easily as good as the very, very best of current UK outline.
Below that you get the Basic athearn,which run nicely, and can be detailed up to make fantastic models at a very low intial outlay,a similar story with proto 1000 and the bachman stuff.
The only exception would be sound. Athearn genesis use MRC sound decoders, which are very flaky on the whole, its often better to buy the non sound version and add an aftermarket sound decoder, QSI revolution and Soundtraxx Tsunami being both excellent and having a wide range of US outline sound available.
The oddball that divides opinion is the Broadway blueline range. They are very well priced, have reasonable sound, but require you to add your own motor decoder. This means you end up with two dcc decoders in one engine, a motor control you fit, and the factory fitted sound decoder. I was sceptical myself, but now have half a dozen of these models and never had a problem.
They run very well, have nice detail, and make all the right noises.(although sound on some diesels is a bit limited compared to some of the more expensive models out there)
