This is a hard choice,
Class 37's the Best british diesel ever made and probley the best diesel around, But you say workhorse well they were in there days but now don't do as much so thats out,
57's don't know much about them apart from the Night Riviera sleeper train, Virgins 57 And WCRC 57601
60's britians most powerfull built diesel, I hear they are doing less work now what they use too could be wrong but I did hear that increased tonnage traffic, Iron Ore, Fuel Oil and Aggregates have seen regular haulage by the 60, which is preferred over the 66 as being slightly underpowered for this heavy traffic
66's well they work most freight workings and I could say they are the Workhorse of the 21st century.
67's are busy on charter trains, and sometimes the odd light freight movement and if it was a heavy movement they have to double header but as you said freight its a no even though I like them as a Modern diesel
92's Well they are Britians most powerfull Loco I think ???? and I also ( Phoenix will hate me after this
) think they are the best Electric train. And man are they loud I could hear one in a moving 375
So its between 66 & 92
I go for the Class 92
