This would really depend on the vertical of your company and the CMS you use. IE are you E-Commerce? Are you using a platform like Magento or Shopify, and if not E-Commerce, something like Wordpress?

This is important both from an feature set integration and economic standpoint. You'll want to find the provider that best integrates with your CMS to maximize its use cases and minimize costly custom development.

A few leading providers are:

Klaviyo, Bronto, Listrak