Really depends on your own preferences. What era of trains do you model? What railroad do you want to represent? i think that the suggestion of an early GP model might be best. EMD designed that engine specifically as a "general purpose" engine to be used, MU'd with other engines, in freight service or for regular switching duties. The engine was also available with higher speed gearing and a steam generator for use in passenger service.
If you want something other than a General Motors design, there are numerous other general use engines like the Alco RS (road switcher) design, the General Electric series of engines that have a "U" in their number, and numerous other company's offerings, depending on how "exotic" you want to get.
As to the manufacturer of the model, Of course, Lionel and Atlas both offer this style of engine. Both are very well made models. Our recently departed Weaver Models had a nice offering of GP engines. I have examples from all the manufacturer's and I'm not disappointed with any of them. All of them will operate on TMCC command control.
Paul Fischer
