--RULES OF THE GAME--
Centurylover68's post initiating this string mentions both "favorite" and "operationally best," but there is a separate "What is your favorite Alco locomotive?" string. I think discussion on this string will be more interesting if we keep it focused on "best," where there are some objective grounds that can be given to support a candidate locomotive model.
-- So, what do I mean by "best"? As a first approximation: the locomotive model a rational railroad manager would be happiest to have on the roster. That's a bit of an oversimplification, though, for at least two reasons.
---Reason 1: By the 1960s it was pretty clear that Alco was on the way out, and no manager wants "orphans" on the roster. So let's stipulate: the model a rational railroad manager imagined to be ignorant of the future of the locomotive industry would be hapiest to have. (If you're familiar with John Rawls's "A Theory of Justice" in political philosophy: this is his idea, asking what a rational person would choose from behind a "veil of ignorance"!)
---Reason 2: Later models usually had improvements over earlier, so if we don't specify WHEN the rational railroad manager is choosing, the question gets biased in fqvor of the C430 and C636! So: that the rational railroad manager would...in comparison to other locomotive types (from Alco and other builders) available at the same time.
There's nothing about being a rational railroad manager that means one HAS to be insensitive to aesthetics! Even on a strict commercial basis, the LOOKS of a locomotive aren't totally irrelevant: a railroad's locomotives are among its most visible assets, and their appearance is surely of SOME importance for public relations: both external (potential customers and general public) and internal (employee morale). But aesthetic criteria would be among the LEAST important (and also the hardest to avoid subjectivity in), so I would recommend that this string stay away from them. (Besides, from a p.r. standpoint, it's probably more important that the locomotives be CLEAN than that they have distinctive sheet-metal work.)
--So: What's your choice? and WHY?