We use Omnisend because of the platform our website is built on and they have an app with easy integration. Before we used Mailchimp which was a winner in terms of audience size and list, but they have been through a lot of changes since those days and hence we opted for Omnisend.
From my personal working experience and the results that I obtained, I would say that Outreach is probably the best tool for creating email sequences and getting quality insights, but it's a bit pricey. Another alternative is to use tools like Mailchimp, Salesmate, or GetResponse as some of them even have free plans, and they also provide useful options and analytics.