Algolia AI Search & Discovery , 30 reviews

I'm a Shopify Experts developer, and Algolia is the best search app for Shopify HANDS DOWN. If you go to Algolia.com, you'll see that they're used on some really big name fashion brands like thekooples.com/us/.

Best part, it's free!!
Their instant search makes it feel like you're on Google. It blows shopify native search out of the water.

Installation is a little more involved than a single click, but it's worth it because this is a real platform, not a shitty half-baked Shopify plugin app as many are.. honestly.

I've implemented this across a few of my client sites. Here are some examples of how you can use it:
http://marissa-webb.com/search
http://chuubie.com/search

Lastly, their support forums are GREAT, they will get back to you next day early morning. and you can search for answers in the forums.

Expert level customizability for your search needs. A++++++++