this sounds like entirely an eye of the beholder and matter of oppinion type question. even what by what measurements to consider best are pretty much an entirely subjective matter.
what can be looked at objectively are fuel consumption to power output ratio's, availability ratio's (i.e. how much shop time does it take to keep it running for how much revinue earning road time)
and so on.
by those two objective ratios, its obvious why the prototype abondond steam. abondoning electrification in north america is a little less clear.
obvously catenary maintainence is a burden, but third rail solves most of that, except, running accross country you then have the problem of small furry creatures and the occasional stray human electrocuting themselves.
best units aesthetically? well again this is very much an eye of the beholder question.
i was rather fond of the oddball uniqueness of the gp-30, still one of my favorites.
well i could go one and on, but i think my favorites have to be narrow gauge dmu's. which of course arn't locomotives as such at all.
there are some i've only seen in pictures and otherwise know little or nothing about, either their opperating charicteristics or performance peramiters.
i like the design with the cab all the way at one end and that end being the front, for maximized visibility. i also like the idea of a sheltered connecting walkway, especially for opperation under mountain and adverse weather conditions.
visibility issues have almost never been adiquitely addressed in locomotive design, not in north america, and little more so elsewhere either.
again multiple unit passinger equipment pretty much takes the cake in this context, as well as that, of to me, over all aesthetics.
one of the reasons i love japanese trains, many european ones, narrow gauge especially everywhere, and favor all three over standard gauge north american primarily freight oriented opperations.
=^^=
.../\...