I can only comment on the ones that I’ve used - Google and Bing, and to a lesser extent DeepL. I use Google a lot when I’m looking at e.g. changeset discussion comments, and as the built-in translation engine on other platforms. I use Bing both directly and as the built-in translation engine here. Google definitely outperforms Bing in my experience - it’s better at idioms that aren’t simply the sum of their constituent words. DeepL is excellent between English and German (it spots issues due to German word order and subclauses that confuse other translators). The last time I used it a lot I found it significantly worse than either Google or Bing when going from English to a non-German language - the results almost gave the impression that they had gone through German on the way (not quite “English as she is spoke” but not great). As a caveat it’s a while since I’ve used DeepL much, so maybe it’s improved.