My favorite is the Atlas Gold

My favorite is the Atlas Gold locomotives.  I've had very reliable operation with them and they sound good.

Scale makes a difference

In HO, I agree with Rick, Atlas China, which for DCC & sound that means gold, is as good as it gets for factory applied electronics.  However the electronics keep evolving, what was great last year is average this year.

MTH impresses me with the overall quality, but they tend to not play well if your only doing DC and in the module control, they are better in their DCS system.

From a pure performance perspective, my tests indicate the Atlas China and anything Kato have performed consistently well and seem to hold the performance with age.  The others that I have tested show more variation.

Atlas & Kato do not make steam in HO, so there are a different set if your looking for steam. I'll let others talk about steam and cosmetic details.

My ratings

1. Rapido - in my view, they have become the premier loco manufacturer if you want highly detailed models with realistic sound.  Of course, they have a limited number of models compared to the others.

2. Kato - still one of the top manufacturers out there with smooth, reliable locos.

3. Atlas - Still very good as well, with a good selection of loco types and road names.

I would also give an honourable mention to Bowser - I've been extremely pleased with the two Bowser locos I have with the LokSound on board.  

If you are looking at budget considerations, though, my list would likely change.  I'm giving what I think is the best regardless of price.

Ratings

Sorry about that I realized I was not specific enough regarding my question. I am talking about HO non-steam engines.

To those that have replied so far, thanks, it great to hear what others say and why they feel that way.

Ratings

1. I never would have said this a year ago, but I think Athearn Genesis has taken the top spot.  Their recent GP38-2 release runs extremely well (as I've heard is true for their GP15-1), and their detail, sound, and paint is excellent.  Doesn't hurt that I prefer the Tsunami decoders that they use over Loksound.
2. Atlas.  I haven't purchased any Atlas units with factory sound, but have converted a number of them to Tsunamis.  Outstanding operating qualities and good detail and paint, though I don't think Atlas is quite up to the Genesis standard in offering road-specific details.
3. Intermountain.  This may surprise many, but my recent Intermountain purchases (SD40-2, ES44AC) have been outstanding.  I'd be tempted to rate them higher than Atlas, but I've heard complaints about tractive effort on the ES44AC.  I heard the same complaints about their SD40-2, but a friend put them up against his Kato mid-production SD40-2 and found that their pulling ability was almost identical.

Personally, I think Kato has largely lost relevance as far as I'm concerned.  Their smooth performance was always their calling card, but that's now been at least equaled, if not surpassed, by all three manufacturers listed above.  In my experience, their detailing doesn't hold a candle to the others.

I'd put my BLI steam

I'd put my BLI steam locomotives at the top for authenticity and quality, but I've seen nothing in Steam that holds a candle to the Bachmann Spectrum Consolidation 2-8-0 with the 1/2A tsunami for reliability and overall useful ness.  Thinking about it, ALL of my Proto steam locos run with the Bachmann's.  All three brands have served me well.

As for Diesels, I have Proto,  Athearn Genesis, Athearn and a couple of BLI's.  They all have the same problem, their wheel sets with the gears eventually spinout. Athearn Genesis with the MRC decoder is an abomination,  the Tsunami tamed the beasts but alas, wheels eventually spinout.  BTW< my layout is always between 65 and 74 degrees.  I hve had a number of Bachmann GP38's's that ran for miles and miles and miles never a failure, but I retired them for era concerns.  Their FT & F7 run well but they do poorly on fidelity.

Joe

My favorites, are the Atlas

My favorites, are the Atlas Trainmasters with sound. Details are fairly good how many others have handles on the hood doors? They run very well and will generally out pull anything else. I have also purchased their GP-7s, and RS-11s  as well as some older SD-24s and like them quite well also. I have Broadway limited RSD-15s as well as SD-9s that I purchased quite a while ago although they are Blue line they perform well but are just a bit below the quality of the trainmasters. I have some older Athearn units and they are models that are around for nostalgic purposes and will fill out some power for consists and will be teamed up with the Broadway and older Atlas units made by Roco. I also have some H-16-44s and some RS-1s made by Atlas and they are very smooth excellent performers. I also have a GE 44 tonner, and a pair of Broadway SW-7s that are excellent performers, paragon series. I purchased some Lifelike Erie Builts and have seen them run but am not real pleased with their coupler mounting system. I will likely be looking at a way to modify them.

Steam locomotives are Rivarossi 2-6-6-6s that are good runners, but do require a bit of attention to get them to the potential they are capable of, 2-10-4s from Broadway Limited, both Blueline and Paragon, 2-8-2s Blueline, and some older Rivarossi 2-8-4s that I might be getting rid of at the next train show. I generally dislike the drivers that need traction tires and I am thinking of replacing them with newer models.

Following the links below some of them can be seen in video action, I believe if you go to my you tube channel which you can access from my blog once you click on the 160 car coal train video you will be able to access others and see 2-6-6-6s running in various videos and you can see on about page nine or later how to tune them up for good performance.

If your not the rivet couter type

It's awful hard to beat the Latest Bachmann Soundvalue offerings for smooth running and good sound out of the box.  They do lack a lot of detail but half the fun is adding that detail yourself.  Having said that my picks are as follows:

1. Atlas Gold DCC with sound.  Very good runners with OK sound. (QSI based)
2. Athearn Genesis with Tsunami sound. Very good running once you modify the electrical pickups on the axles.  (shouldn't have to do that on a $300 locomotive but it's needed)  Sound is very good overall.   Tsunami decoders suffer from poor horns and there are better decoders out there if you want good horns.
3. BLI with Paragon 2 sound.   Smooth as silk but again suffer from electrical pickup issues.  Modify them the same as all Atlas/Kato/Genesis and you will get good performance. Fantastic horns sound like you are trackside!
4. Honorable mention:  Bachmann SoundValue line.   As far as performance goes, they will hold up to the top 3 already mentioned.  As far as sound, hard to beat for the price although limited functions, very respectable.  As for detail, well let's just say, "what detail"!  Adding your own detail will make a very nice model that can be handled with out fear of breaking something. 

This is my take on these.  I'll be honest in the fact that I have a Blue Box GP40 that has been extensivly reworked and runs as well as the above mentioned and the sound is phenomenal!