I must admit I'm a bit partial to the replacement, the DD51, even though it will never win any beauty contests.
I saw one running once, it was pulling a nostalgic trip set of cars for sightseeing Omura Bay. Its a practical, clean design.
I'm more into the steam engines, the 8620-series is my favorite. I saw a documentary on the retirement of SL Hitoyoshi, the last operational 8620, and you could see how attached the engineers and maintainers got to it. The engineer explained that a steam engine is more like an animal than a machine, it needs your constant attention (and feeding) in ways modern engines don't. (JR steam engine engineers are chosen from experienced regular engineers) The maintenance men and those that did the boiler replacement kept referring to it as a living thing. There's no manual, you have to feel it with your senses to know what it needs and that only comes with experience. The retirement ceremony was ended with everyone bowing to it and thanking it for it's service. I believe it's on display at the Mojiko train museum now. I got to ride that one a few years ago along with another 8620 at the Kyoto Railway Museum.

I wrote about it here:
https://www.wayfarerdaves.com/riding-the-rails-on-sl-hitoyoshi/