Scale: HO. Cars, engines and accessories are easier to find. And when compared to some large scales, easier on the pocketbook.
Manufacturer: Many. I look for quality and detail. If the piece is then affordable and worth the asking price, I buy it. Much of my rolling stock is Athearn. These cars are very affordable and look good. Engines are Athearn Blue Box(converted to DCC), Spectrum, Proto 1000 and 2000, Genesis, Broadway Limited, Rivarossi.
Model: I like them all or I wouldn't have bought them. I prefer steam engines and early diesels, the F-3's and 7's, early Geep's, the Alco's, PA's, Railmasters...... Steam? If it has rods moving, I like it. My modern stuff is pretty generic. Most made by Spectrum, Proto 1000 and 2000, Genesis, Broadway Limited, Rivarossi....Higher quality, and price unfortunately.
DCC has been the big story in the hobby in the past 15 years. Innovations in tooling have also brought better detail and quality to the product.