I think it has to be a 'successful combination' of all of them. Email campaigns, is about what you want to bring to readers. How you inspire them on your products and how you keen their willingness. You have also to consider what is the main aim of your campaign and who do you target. So, in most cases, you find something ready (from theme forest templates, or from mailchimp or any other email client that support email templates) and your designers has to transform it to make it suitable for the current purposes of your email campaign.
In every case, you have to run A/B tests. You have to learn each time from every past email campaign you have run, so to make your next campaign, better according to goals you set. A short example on this, is the following case. You have run a campaign and your Click To Action didn't reached from your recipients as you expected. You have to test, in which way (if you change the position of the banner for example) is reached in maximum
I think it's not the design that makes the difference as is how you put your content and how you present it in an email to make the readers to want to learn more, to reach your products and how you guide them as well after reaching the email.
I believe that can help you if you study how some companies send their emails (ex. Apple's campaigns are light and simple enough, but keen you to see more, even if you already know what is about, and this is incredible).
For templates, you can always find great ones in themeforest and if you have creative designers, you can transform them to great email campaigns.