It all depends on what satisfies your requirements. Algolia is quick and easy to get up and running. It has a free plan, plus there's no need to host it yourself. Great for all purposes.
Scout has other unofficial drivers, like ElasticSearch: https://github.com/babenkoivan/scout-elasticsearch-driver
By far, the two most popular services are Algolia and ElasticSearch. Each offer similar plans, but elastic also has a powerful API and allows for self-hosting, unlike Algolia (don't quote me on that).