Though the quality of an email can be assessed to some extent, there is no such thing as "the best" when it comes to emails and email marketing.
Suppose a company can interact with its trial users, educate them about its product(s) and how to utilize it more efficiently, and turn their leads into customers in the end. In that case, they have the best free trial emails!
But still, if you want to see some creative and well-structured trial emails, you can check them here.