While all elements of the email are important, you will want to focus on the subject line.
If people don't like what they see and are not enticed to click, they simply will not open your message. On the other hand, if your headline is attention-grabbing, they can't help but click on your message to see it. Once they've opened your email, it's going to be that much easier to build your relationship and make sales.
Here's a great resource that contains 20 neat headline generators.
What's important is the inputs on the tool you ultimately use. After a while of using it, you'll just know how to write compelling headlines.