DuckDuckGo about 60% of the time, followed by StartPage and Google. The main reason for choosing DDG is privacy. And it's the same for StartPage too. I never search on Google when logged in, and I try to isolate browser activities in different ways (on Firefox, I clear cookies with Cookie AutoDelete, use Containers, etc.).
I usually start my searches on DDG, and move on by modifying the same search query with a !s prefix (for StartPage) or a !g prefix (for Google). DuckDuckGo's date search is comparatively limited. DDG still isn't adequate for me for tech related searches, though it's gotten better over time (StartPage, which is a proxy for Google, and Google itself, are much better). Image search also has more refinement options on Google than on the other two.
I also use DDG to search Wikipedia (!w prefix) and IMDB (!imdb prefix, which is a bit long to type though).
DDG has a bang search keyword for almost every site you may want to search on. [1]
[1]: https://duckduckgo.com/bang