I use Mailerlite, mainly because it offers a really competitive price for 20k subscribers. Not perfect but delivery rate seems good, subscriber handling is powerful, comes with wisywig editor and templates, integrated sign-up forms for your web site (widgets) and a one-off annual fee is much less than mailchimp and others for the volume. I think it starts free for mailing < 1000 people (or used to)
Mailchimp is easy and scalable. We used to use it for montly mailings to about 10k addresses, before we moved everything to Salesforce and the filthy, unmanageable beast that is Salesforce Marketing Cloud.