We're looking for the "best" from the point of view of a railroad operating them. (Wecould be looking forbestfrom a railfan's point of view, but that would get us into too many subjective, aesthetic, issues ... and some of us like things just because they are downright quirky!)
(i) Introduced in 1954 or later: we wnat something with a 567C or later version of the EMD engine: it took that long to fully debug the design.
(ii) Hood unit. Outside North America, where axle loadings aremore of a concern, the boxcab or truss frame design allows for lighter weight locomotives, but in North America the railroads voted with their purchasing dollars for what they thought was most useful.
(iii) Probably a "second iteration" of any technological advance: early turbo-567 units were problematic (some GP-20 were de-turbo'ed by around 1970), the 40-series had enough defects that after a half-dozen years they underwent a major design revision to get the Dash-2 series.
Sales statistics ought to be an indication of what the professionals thought. Which would suggest the GP-9, the GP38-2, the SD40-2, and the SD70 as candidates.