You will get a million different answers on this question. While some people will rate a manufacturer based on the availability of products for the era they model, I believe a fairer assessment is in the quality of their products and their performance on the rails.
Im sure Ill leave some out, but here's my ranking from in order from first to worst:
1. Intermountain. Kadee couplers factory installed, factory installed metal wheels that are the standard for the hobby, lots of nice detail, good weight. You pay a bit more, but you get what you pay for. 5 stars.
2. BLMA. High quality, excellent detail, metal wheel, knuckle couplers, grab irons, decent price for excellent quality. They seem to never disappoint with their products and quickly sell out.
3. Exactrail. Very similar to BLMA, although sometimes a little bit more. Nice rolling wheels, incredible detail, usually kadee couplers.
4. Atlas. Sturdy, good detail. Come with metal wheels that I sometimes replace, and with Accumate knuckle couplers that I always replace.
5. Kato. They are not a big player in the HO rolling stock market, but what I have of their's runs well. Namely a couple of sets of the Maxi IV well cars. Nice detail. I replace the couplers with kadees bc I find the stock Kato couplers dont play well with others. Ive heard their hoppers are nice.
6. Walthers. Gold and Platinum Line. Although they come with metal wheels, I replace them with Intermountain wheelsets bc they run so much better. The New Protomax couplers are nearly an exact clone for the Kadee coupler #5 and work well. The wheels on these models and the weight of Walthers products can be hit or miss. Nice quality overall and decent detail. I have alot of these. The Proto 1000/2000 cars are popular too.
7. Kadee. I dont have any, mostly bc most of it doesnt fit my era. But they offer alot for the price. Made in USA.
8. Athearn/Athearn Genesis. Again, I have alot of these bc of the selection/sheer amount of product they put out. I have found the wheels, although metal, can have bad flanges or be out of gauge. Sometimes alot of quality control issues. I find the stock McHenry couplers to be absolute garbage and always replace with Kadees. Seems to me, that the new RTR models are the old Genesis models at the same price without the spinning wheel hub.
9. Atlas Trainman. IMHO, a tremendous value. Good detail, and you can easily update the couplers and wheels and grab irons if you want.
10. Accurail kits. I dont have any of their RTR products, but the ACF 3 bay covered hopper kits are decent. Easy to build, affordable; I replace the wheels with Intermountain sets and add Kadees. Not the greatest of detail, but the run well.
11. Athearn Bluebox. The ubiquitous HO kit car. Very customizable. You cant go wrong. Detail and looks depend alot on how long ago it was made. I dont have many, but the ones I do have, Ive upgraded the wheels/couplers.
12. Roundhouse. I like the detail on these and with some metal wheelsets and couplers you have a nice car.
13. Bachmann. Decent looking paint jobs that seem to be either very flashy for what you see actually on the RRs or very bland. The stock couplers are junk.
14. Lifelike/Tyco/ahm. Train set quality. Look plasticy and cheap. You can do better.