Below is a few HTML email template I found with their stats as of 28 Jan 2018.
|Name||Github Star||Issues||Last Release|
|Really Simple Responsive HTML Email Template||6139||3||Aug 8, 2017|
|Responsive transactional HTML email templates||5727||6||-|
|Cerberus Responsive Emai||2727||4||Dec 24, 2017|
Personally I use Really Simple Responsive HTML Email Template's email-inlined.html (not marketing email, just simple hello or reminder email).
NOTE: You can SendGrid Template Editor (need to create a free account) which allow you export (
Design -> Build -> Advances -> Import / Export) the email template.