For the "first generation" of major dieselision, there is no doubt that the EE Type 3, later class 37, was the most successful on many counts, and this is best reflected in their longevity and the usefulness the class offered into the 2000s.
The only other comparable locomotives are ....also English electric, being the venerable 08 and the class 20, although the latter is most often used in pairs, a somewhat wasteful formation both at the time to erradicated steam and later in their lives. However, they too prove to have their niches.
The main failings of the first generation type 4s were the 1CoCo1 wheel arrangement and the overall wieght. Coupled to the class 40 and 44 being underpowered for their intended duties, and the lack of reliability from the class 45, then these locos were a heavy, expensive experiment. The class 46 was reputadely a more reliable unit, as were of course the class 40s, some of which were in completely working order when cut up in 1985-6.
Of the second generation, there were only three classes eventually ordered: 35 being the first, then 47 and 50. Once again it was actually the type 3 which prove to be the most reliable and "designed for task, delivered to design" . Both the 50s and the 47s required extremely expensive refurbishment to overcome their reliability issues, and in effect the 47s in particular did not live up to their design missive.
Brush Traction then had a pragmatic approach to what was still fairly experimental machinery. They had already shown this in re-engining the problematic and underpowered type 2 AIAs, and took a sensible decision to de-rate the new type 4s after rectifiying the terrible problems with the Sulzer units. One of the key issues was most likely caused by sulzer sub contracting production of the LDA series engines out, and thus quality control at various levels was not fulfilled.
In life, however, the sulzer 12LDA twin bank engine prove easy to both rectify and "keep on top of" with regular, specific maintainance. It is a single turbo engine with otherwise good access to components and inspection points. This contrasted with its own cousin, the 12 and 16 LVAs which had "blind" spots in the crank cases which lead to catastrophic failure due to a simple locating pin tool being ill machined! The 12LDA units could be hammered back into service rapidly, and given the amount of idle service duffs seemed to get in some areas when the more powerful 56s and 8s came in or when pairs of class 37s or 31s were needed to start heavy loads the 47 couldn't cope with in its solo existence, then class availability was kept artificially high. A better measure is MPC: miles per casualty, and for the same high speed services as the 50s, the 47s faired no better in fact!
The 47's legacy really lived on because Brush won the lions share of the two next generations of locomotive orders in the class 56 and 60. The nickname "duff" has cast a long shadow in UK locomotive history.
In the mean time however, GEC had gone into Alsthom and had concentrated on mainly AC locos for mainland europe with only relatively small orders for the "prima" units, tha majority of those for France and the UK, with the rebuild of the 333s being just that. It was the USA who had kept advancing diesel loco technology in heavier RA7 + freight locomotives, and actually could offer units down to under 120 tonnes based around the GM 645 line of PUs and CoCo format.
The failure of the 56s to haul the heaviest trains in the UK, lead to the class 59 outshining all but the "tugs" when they came on the scene. Major teething problems once again with the 60s meant that in the soon-to-be private world the EMD class 66 would rule the rails. ( although as a footnote, the USA proiduced Class 70 has had its share of teething problems)
So in a way, the perpetual under-delivery by Brush and compacency by GEC Alsthom opened the gates for the very competent N.American GM-EMD to swing in. WIth the Class 60s likely to be withdrawn or rationalised, we will one day soon see the end of UK produced locomotives on British metals.