Best option for long-term development

Mautic is not Salesforce or Dynamics of course; it is hard to install, still buggy and you have to know what you're doing if you want to use it. But that is your admission fee to an always free system that does essentially all the same things as the pricey alternatives, does it effectively and gives you total control. Mautic was acquired by Acquia, the company behind Drupal, and has been making substantial upgrades since that seem to be accelerating. The system has an active user community, active development and will only get better, making it an excellent choice to grow with and build your own systems around and never be dependent on third parties.