Most of the general comments I would add have already been covered above, so I'll cut to the chase from me.
HO locomotive brands that I would not hesitate to buy if I saw something I liked or needed:
-Atlas Master Line
-Atlas Classic Gold or Silver
-Walthers Proto
-somewhat newer Proto 2000, i.e. made in say...the last 20 years or so. I think the detailing on some of the early P2K from the 1990s wasn't as good as the later years, IMO.
-newer Athearn Genesis, also within the last roughly 20 years, same comment as with Proto 2000.
-Kato, although I prefer DCC and sound, and it seems rare to find Katos with that installed. I have done DCC conversions on Katos and they run absolutely great; however I find the detail on their locos made 5-10 years ago is just a shade below the level of a Proto or Genesis right now.
Other brands that I've been content with:
-newer Intermountain, they seem to have come a long way from their early products from 20 or 30 years ago that I still see in the back room of my local hobby shop.
-Athearn RTR, I have a few of these mostly when it's a subject not available elsewhere. e.g. I recently picked up an ATSF GP60B by them since it's a less common locomotive.
-Fox Valley, I have four GP60s from them, converted them to DCC with sound, and they run and look great.
-I have one Bowser Alco C636, it's the only thing I have from them. But it's been great, runs well, looks good, and even has working 3-color classification lights. However, chatting with the owner of the LHS, he said he bought one and had problems with it, ended up having to send it back for repairs.
-Atlas Trainman Plus and Lifelike Proto 1000, I'm not sure if these are even made anymore but they provide a decent level of detail at a lower price point than most of what I've listed above.
-I have a few Broadway Limited steam locomotives. You were asking about diesels, so I can't really weigh in on BLI diesels. But in terms of steam locomotives, I have two BLI steamers that run great, and I have one that has problems.
I don't have any Scaletrains or Rapido locos, but someday I'll probably give them a try. The few Rapido passenger cars I have are nice.