The first key to any good operating model railroad layout is good trackwork. The second key is to have decent locomotives that are dependable and run smoothly. Your entire experience with model railroading is to have engines that operate like dreams. It is better to have one really good loco that does exactly what you want it to do than it is to have five that don't run well and are frustrating to operate.
Good locomotives, unfortunately, are not cheap. Nevertheless, they are an investment into your future as a model railroader. This is not a place to skimp. The models listed here are handpicked to be the best of the lot, based on personal experience or on customer reviews.