I agree that you just can’t go wrong with Atlas. They also have the largest selection of available locos and road names – with more coming out all then time! Sadly, with my limited budget, I only own a pair of SD35’s. They’re great runners (made in 2000), but the current generation with slow speed motors is even better.
I have never been able to afford Kato, but I have seen them in action and they are known for their superior silk-smooth mechanisms. Sort of the BMW of the N-scale world. But I noticed they tend to model more modern locos in limited road names (mostly western as of late). They also seem to sell out quickly, and for a premium.
Lifelike is close, if not equal in their current releases to Atlas. I own their SD7’s, which are pretty nice runners considering their somewhat older design. I also own their GP20’s, which are outstanding – especially for the price. The GP20 mechanism is actually slightly smoother than my Atlas SD35’s! They also look very similar on the inside…
I am hesitant on Bachmann. I’ve heard great things about their 2-8-0, and some decent things about their current crop of spectrum diesels. Maybe it’s just my own personal prejudice, but my image of Bachmann comes from my old U36B tearing around my layout at warp 9 last night to the delight of my kids. Hey, I guess at least it still works after 20 years…
Intermountain is a newer player, and I’ve seen their F3’s up close and in action. Very nice, and on par with Atlas. I’ve also heard great things about their FT’s and TM’s. Certainly worth trying out.
MT is also a newer player, but they only have an FT on the market (that I’m aware of). I heard it was pretty good. Not much else.
MDC/Roundhouse I have no experience with, but have heard good things about. Steam only.
Con-Cor, Model Power. I have not heard great things about them.
Athearn is another new player. They only have one loco on the market right now (F59PH). I’ve heard so-so things. However, they have announced SD70 series, so they are certainly worth keeping an eye on.
Well, that’s all I know (or have heard about). I'm sure others will chime in with their thoughts & experinces (which will probably be more than I have).
I also have a website that I refer to for reviews of the older N-scale locos:
http://www.visi.com/~spookshow/locos.html
Hope this helps.