I don't think the FSF would promote any particular search engine because they
are services, outside of a user's control. But they all work fine with free
software.
RMS says he sometimes uses DuckDuckGo, as well as other search engines. [1]
I think DuckDuckGo and DuckDuckHack is good, because they make some source
code available. I have tried to access YaCy a few times but it either timed
out or the results weren't that good.
When DuckDuckGo doesn't work (it seems to not like quotes in a search
sometimes) I use StartPage. Although I used someone else's computer and I
think they might use Google Ads (I have JS disabled so I don't see them, but
they might have changed by now).
I don't object to using other search engines, but I generally don't use them
either. As long as they work okay without JavaScript and cookies.
