What is the most popular email newsletter service?

If popularity is ranked by likability - the most popular email newsletter service is different for many people, and rightly so. Preference of the newsletter's topic differs greatly among web surfers. While some services produce an Excellent newsletter on one topic, other services may produce a much better newsletter on a different topic. If popularity is ranked by sheer volume alone - ConstantContact is a very large one with a large selection, followed by CheetahMail and MailChimp, as well as several others.