I can answer this form a conventional standpoint only. I presume you understand 'convetional', but if someone is later researching this topic, perhaps it is a good idea to define it. Conventional = YOU control the operation fo the engine - no "systems" of digital elements are used. Think of it another way: 1950's style train running.
These are brands I have:
MTH - great trains. Even PS-3 can be run conventionally, just not all features can easily be activated. Great rail cars in the RailKing (the yellow boxes) and Premier (purplse boxes) lines. RailKing is semi scale, typically, and Premier is scale sized and more detailed. Quality very good on both lines. Engines are tanks.
Williams - excellent value. I have Williams by Bachmann (WBB) and pre-merger Williams (when it was in Columbia, MD). They are like Hondas. Always work, easy to fix (if needed), and a good value. Many dealers selling older styled inventory (a.k.a. old tooling) for good deals. Even the new tooled items are great sounding and not terribly expensive. Trainworld selling the newer engines with Tru Blast II for around $250 +/-. True Blast II is the newest generation of sounds to Williams, and they sound pretty good. Willaims cars, I pass on. Too much the 'toy' size, what some call O-27 scale; not detailed either.
Weaver - most of my Weaver engines are 10+ years old. Tanks. Well made. Their cars are excellent, but if buying used units be aware that they might have plastic trucks. Currently made units, I believe, are all metal trucks. Quality and detail are excellent.
ETS - these are Czech republic made trains that look similar to tinplate. They are nicely made in passenger and freight. I have one set of each. Reliable. Nice runners. The ones I have come with European couplers, but some are now made with knuckle type couplers that we see commonly here. But the engines and cars are older style - not like the modern, Euro trains MTH sells. They are a great value in my opinion and a nice conversation piece.
Hope that helps.