What is the best way to send customized emails? Which add-on would you suggest?

Best, unfortunately, is really dependent upon your needs and your budget.

If you're looking for a solution that's tightly integrated with Salesforce, and provides some powerful advanced features, look at Salesforce Marketing Cloud. If you reach out to your account exec,

he can provide you with pricing and a demo.

Beyond that, there are a number of tools that don't integrate as well, and offer fewer features, but are cheaper and allow for customized mass emails. Mailchimp is a common solution. The integration is klunky and one-way, but allows you to send based on your Salesforce data.