With the release of Tor Browser 6.0.6, we switched to DuckDuckGo as the primary search engine.
For a while now, Disconnect, which was formerly used in Tor Browser, has had no access to Google search results.
Since Disconnect is more of a meta search engine, which allows users to choose between different search providers, it fell back to delivering Bing search results, which were basically unacceptable quality-wise.
DuckDuckGo does not log, collect or share the user's personal information or their search history, and therefore is best positioned to protect your privacy.
Most other search engines store your searches along with other information such as the timestamp, your IP address, and your account information if you are logged in.