You can decide the best product search and filter app by looking at some metrics:
1. Which features do you want for your store?
Do you want a basic instant search and filter or more advanced one with other features such as Search merchandising, Synonym search, Internal search analytics?
For example, Walgreens display a promotional banner on the protein bar page to promote their marketing campaign.
2. Check if it's optimize for Mobile
Filtered navigation was initially designed for desktop and laptop users. It becomes more challenging to translate this experience to mobile because the screen is not large enough to show a full set of filter options and results simultaneously.
Remember to check the mobile UI. Most eCommerce use overlay filters on top of the collection or search results. This dynamic interaction design works great because users can always see which filter they are applying and access easily to further refinement options, all without leaving the context of the collection or search result screen.
Even Wallgreens fail to provide a smooth filter function on mobile. Their product filtering on mobile automatically collapsed after users select one value. If I want to select multiple brands I have to go back and forth to apply different filtering values individual. Moreover, the filter option drop down makes it hard to navigation to other options below.
You can look through the search and filter experience on our demo store at https://usf-vertical.myshopify.com/ using Ultimate Search and Filter https://apps.shopify.com/ultimate-search-and-filter-1
@George_Greenhil: I would not compare BoostCommerce with Searchanise. Searchanise is better in term of functionality. However, the Ultimate Product Filter + Search is the best choice (features, theme support, etc.) but it's the most expensive one. You get what you pay for!