Which of the following is a measure of the effectiveness of your emails as part of HubSpot's analytics?

The measure of effectiveness that best reflects how well your emails are performing in terms of user engagement is the Click-through Rate. This metric indicates the percentage of recipients who not only opened the email but also engaged with it by clicking on one or more links within the content. A high click-through rate suggests that the email is not just reaching the inbox but is compelling enough for recipients to take further action, which is often a key goal of email marketing campaigns.

While delivery rate is important as it indicates whether your emails are successfully reaching recipients' inboxes, it does not provide insight into how recipients are interacting with the content. Similarly, open rate measures how many recipients opened the email, but it does not reflect the level of engagement beyond that initial interaction. Unsubscribe rate shows how many recipients opted out of receiving further communications, but it does not indicate positive engagement. Thus, among these metrics, click-through rate stands out as the most effective measure of engagement and success in email campaigns.