The question is not which is "the best", but which fits your text type, your languages, and your budget. All four are integrated into WP-Translate and you can switch from one to another without losing anything translated.
DeepL
Strong in: long, nuanced European texts. German, French, Italian, Dutch, and Polish come out especially natural. It is the one that best preserves the tone of a carefully crafted commercial text.
To consider: covers fewer languages than Google. It has a generous monthly free tier to start.
Choose it if: your market is Western Europe and the text is brand-oriented.
Gemini
Strong in: understand the context. Being a language model and not a pure translator, it better respects HTML, proper nouns, and technical terms, and follows instructions ("translate this as button text").
To consider: it is somewhat slower per string than a specialized translator.
Choose it if: you translate technical content, formatted datasheets, or texts where context matters more than literalness.
Google Translate
Strong in: coverage and speed. More than a hundred languages and very low latency, ideal for huge catalogs.
To consider: in brand texts, it falls flatter than DeepL or Gemini.
Choose it if: you have high volume, unusual languages, or are in a hurry.
Yandex
Strong in: Russian and Eastern European and Central Asian languages, where it usually outperforms others.
Choose it if: that is your market. For Western Europe there are better options.
To the point: you don't have to marry one. You can translate the catalog with Google for volume and redo the twenty important pages with DeepL or Gemini.
How to control the cost
All charge per translated character. Three things reduce the bill much more than choosing the cheapest engine:
- Translation memory. Repeated strings do not go through the API again. In a store, this is the biggest saving item.
- Translate in phases. Start with what is seen and what sells; the old archive can wait until next week.
- Glossary. Avoid re-translations to correct proper nouns.
And remember that the API key is yours: you pay the provider directly, without intermediaries or margins. That also means you can see the real consumption in the provider's panel and adjust it.