Which email template builder do you use and why?

I am curious as to which email template builder you use and why? I have always coded my email templates starting from a free basic HTML template and edited some of the HTML manually to customize it for my needs.

But is there a better way to do it?

I would be looking for something where I can create templates with images, gifs, and then define placeholders that would be swapped say by making a post request with the actual data and and being sent back the full template as a sting that could be converted back to HTML and sent via one of the most common email providers(sendgrid, mailchimp)?

Does that exist?

Thanks in advance for your help.