Okay, just stumbled across this site and I'd like to point out that those of you who say brass model detail exceeds that of plastic are not really correct. That brass models are in excess of the majority of plastic models (or even more so die cast variants such as MTH due to in arguable material limitations) is most often the case. But even with Athearn's QC issues of late their tooling on the S.P. MT-4 exceeds any brass model ever produced.
There isn't a brass model steam locomotive that has scale cross section spokes on the drivers, nor anything but a relatively crude bell yoke and other lost wax castings, or the cross-sectional thickness on the cab roof overhang. Those are some of the most obvious comparisons, do look for yourself. But to be really truthful, each of you needs to do as CNJ999 has recommended: "take a really close look." Personally I'd recommend using a 14 or 20 power jeweler's loupe for that close of a look. I use both.
Brass models that have large area stampings (such as first generation diesel noses) when scrutinized under this level of magnification appear just as do old tinplate toys, stretched, rippled with distortions, etc. No joke. But you say "that's a ridiculous, no one looks that closely!" Perhaps, but once you've seen this for yourself you will see it with the naked eye from then on. On steam, you'll see all of those "rivets" are in fact round and flat discs, nothing like a real rivet. It goes on and on.
The way in which plastic tooling is created can produce (but rarely is done to this degree) "State of the Art" results well in excess of any brass models. As an example go ahead and use those same loupes on an Athearn Genesis F Unit. There you'll see accuracy in replication of the prototype that you were completely unaware of. No short-cuts, a fineness of detail that is impossible to render in brass due to the grain of this or any other metal. The breakthrough in the old analog record industry came with vinyl 33 1/3rd RPM recordings replacing the pre-war shellack 88rpm's due to the fineness of grain of plastics over any other substance. That literal lack of grain found in plastics is one of that family of materials strongest assets.
So regarding the above comment; "Whats the most detailed HO models available?" First I'd add to that question the word "accurately" as in; "What's the most accurately and well detailed HO models available?" This as opposed to the total number of more or less indiscernible blobs representing detail is a somewhat more meaningful question.
I'd strongly recommend an extremely close scrutinization of the above mentioned two Athearn model series. There is simply nothing else like them. I still buy brass models, but only on rare occasion, as they're relatively primitive.