Probably better in the N scale forum this one, but regardless, I have rolling stock from Atlas, Athearn, Kato, Roco, Trainworx, Walthers, Bachmann and InterMountain. For passenger sets (Metra, California Zephyr, Super Chief) Kato cannot be beat, but for freight car range Atlas is on top (from what I've seen anyway). I only have a few things from the last five names listed but they don't really have any problems.
The most problematic for me has been Athearn. I only have a handful of freight cars from them but nearly all were poor rollers, and McHenry couplers are an abomination which I have mostly replaced.