I have to agree...nicest may have different meanings to different users of a number of locomotives.
For me:
Brass = the nicest. I have one brass steamer due to me very shortly, my first order. I won't get another soon.
Taking brass as the standard, which engines are the nicest to look at in plastic? Proto 2000 by Walthers have always been my gold standard. However, when I got a Hornby/Rivarossi C&O Allegheny three years ago, the P2K standard was met by that one example. I can't vouch for other Rivarossi engines.
Broadway Limited Imports (BLI) are also nice, although they would get about a B+ from me for details and niceness across their production. There is some variation, though, and a few of their steamers are really something. Take the Hybrid steamers, mostly brass with metal and plastic details soldered or glued onto the frame and superstructure. Their Union Pacific 2-10-2 is one such engine, and the Q2 seems just as 'nice'.
Brass has had some problems with running qualities, but not so much in the past few years. That, to an operator who doesn't just collect display brass, is a huge part of 'nice'. Modern plastic and metal combos seem to have a good reputation, and if you have problems, the warranties work well.
I would be remiss if I didn't mention the Spectrum Line from Bachmann. They get good marks from many who use them. Bachmann's warranty is the best in the industry....lifetime for the orignal purchaser of any of their locomotives. The Spectrum engines run well and they are highly detailed.