Quality manufacturers vary by scale, era of model and personal preference. I model modern diesel in HO, so my recomendations are for this type of equipment.
I avoid Tyco, Life Like and Bachmann. Quality engines are available from Atlas, Atlas Trainman, Athearn, Athearn Genesis, Kato, Proto 2000, Intermountain and Stewart. Quality rolling stock is available from the same manufacturers as well as bowser, accurail.
I recommend Code 83 track. Track from different manufactures will work together. Micro Engineering has the best flex track, but it is harder to work with then the stuff from Atlas. Walthers has a nice line of DCC ready switches, but availability is not very good. Atlas makes some code 83 switches, but the selection is not as good.
I don't use products like e-zee track with plastic ballast attached to the rail as there is less freedom in what you can do.
There are many manufacturers for buildings and bridges. Walthers has a great line of buildings and bridges. They also make the nicest turntable I've seen. 130'! They also make and smaller 90' one. Central Valley and Micro Engineering also make nice bridge kits.
These are, of course, just my prefferences. Choose what looks and works best for you.