Updated: March 11, 2018
There really is no best answer for this because you really have to make due with the tools that you have available.
ConvertKit and Drip are popular choices but they do require a monthly fee so I would suggest only using them when you feel like you will need their features.
How will you know when that is? When you learn more about email marketing and discover that you want to do something that isn't currently available in the email service provider you chose.