Defining the best mobile search depends on what your needs are. Do you want search primarily for the web? Or to find stuff on your phone? Most mobile search engines combine both web and device results, usually with a prioritization of one over the other. Essentially, it comes down to finding the right one for your needs.
Google Now is obviously the best if your priority is searching the web. It has great voice recognition software and can give you web results for basically anything.
(available on Android )
Quixey takes a different approach in their Launch app by putting their search results on deep view cards. “Show search results that are functional and dynamic, not just blue links."
(available on Android)
Spotlight Search is a great device focused search, with deep links into app search results.
(comes standard on iOS)
WhereDat focuses on device search. Searching everything on your phone, it supports search into Gmail, Slack and Salesforce, with more to come.
(available for Android)
Note that this post was originally posted as a Quora answer.