If you're looking for conventional, I don't think you can go wrong with Williams/Williams by Bachmann. No frills, including no steam or diesel sounds, just a bell and whistle/horn. Every one I've purchased has run well. I have 2 of their NW-2 switchers and 5 of their O27 steam engines. I like their 4-6-0 Ten Wheeler best and have even thought about getting another, the nicely colored C&O. It is geared really well. It's also a bit smaller and looks fine on 27" curves.
I don't own any other W/WBB diesels, but for your size layout, I'd go with the smaller FAs over the F3s. They look more at home on 27" curves.
If you want to pay a little more, then I'd recommend either MTH RailKing or Lionel LionChief Plus. These will bring you handheld command control, sounds, and some other niceties.
In any of these cases, make sure they're rated for 27" curves, not 31".