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 |
| SendGrid email-templates | 221 | 5 | - |
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.