I'm new-ish to N scale and only own 3 different brands, so take this with a grain of salt, but of the 3 I'd rate Kato first, Atlas second and Bachmann third. Here's why:
Each Kato has performed flawlessly out of the box, pulls well and runs quietly. I couldn't tell you if they're prototypically accurate or not but, not being a rivet counter, they look good enough to me.
My wife rode the Super Chief and just loved it, so naturally when we ran across an A/B set of F7's they wound up coming home with us.
That was over 20 years ago, and they sat in a box until this year when I finally got around to building a layout.:dunno:
Kato re-released that same set earlier this year so I picked them up and sure enough the new units look and run exactly the same as the old. 3 of the 4 run at exactly the same speed, with only one b unit running a smidge faster than the rest.
My point being, while they may or may not be the best, their quality certainly appears to be consistent.
At sane speeds my SD70 will negotiate the tight curves on my layout, and the little C50 (a Japanese model) pulls far more than it should for it's size and weight.
One last important difference-None required breaking in to run smoothly.
My experience with Atlas is a little different. I only own 3 of their engines, all C420s. One runs a little quieter than the Katos, another about the same, and the third is much noisier. They all run at vastly different speeds but it doesn't seem to matter much when they're lashed together. The third, (purchased separately) arrived DOA and had to be sent back for replacement. Still, I like them and wouldn't hesitate to recommend the brand.
And then there's Bachmann. From what I've read they used to be considered junk but have improved in recent years. I don't know if that's true but the models I have are definitely not on par with Kato or Atlas. The 2 diesels will not run smoothly at low speeds and one is loud as h e double hocky sticks. The 0-6-0 broke an axle at less than 20 hrs run time and never ran well to begin with. The K4 actually runs great, very smooth, very quiet, but when starting from a complete stop, STOPS every time it makes the 'chuff' sound. (once it gets going this doesn't continue)
The Baldwin I just purchased is going back to repair a nasty wobble.
So yeah, not a huge fan here. BUT, when it comes to choices, Bachmann is king-nobody else offers as extensive a line-up. And since they're cheap I'll continue to take my chances with them. But I wouldn't recommend them to anyone else.
Unfortunately we're very limited for steam choices. Bachmann of course has the largest selection, Atlas currently has none, and Kato offers exactly two, which are the same except one is a freight version and the other passenger. Broadway limited has several that appeal to me visually but they get such mixed reviews I haven't been willing to drop the kind of coin they want.
I agree with gimme30 on the Kato brand. They just can't be beat. My two PA-1s run really well. Of course every manufacturer builds a lemon now and then, but just about everyone on this forum agrees about the good quality of Kato. And, by the way, everyone here likes Kato unitrack too.
I have no experience with Atlas locos.
I have a Bachman 2-6-0 steamer and an RS-3 diesel, both with DCC. The 2-6-0 runs well, but one axle on the tender kept falling off. I wrote to Bachmann, and they quickly sent a new truck for it. The RS-3 runs very well, and creeps amazingly slowly. Both are quiet.
I have level track, so can't speak much about their pulling power. All do well with moderately-sized trains.
I have been in N-scale for many, many, years. In all those years, I've never encountered a bad Kato locomotive. I recommend them highly!
Kato does not make many steamers, but they have a good selection of diesels. Their F-units, at least the earlier DC models that I have, are noisier than their other locomotives, but they run well, and look good. running them at slower, more realistic, speeds quiets the noise considerably.
Their 2-8-2 Mikado Steam locos are great runners, but I found they are not comfortable with curves tighter than 16" radius. They will go around tighter curves, but they tend to derail often enough to irritate me. I changed from a 12" minimum radius to 16" because of this problem.
FYI: Some "Atlas" locomotives are actually made by Kato. Others are not, but generally Atlas locos run well.
I don't buy Bachmann. I remember the old Bachmann junk too well. Newer Bachmann locos are said to be much improved. Still, even now, whenever you see a post about somebody having problems with a locomotive, it's usually a Bachmann. I just don't trust the brand.