Our company has learned a lot of lessons through A/B testing various components of our email newsletter. We started by testing subject lines, then pre-headers, then calls-to-action throughout the content of the email itself. Each of these tests has given us new insights into what our email subscribers are looking for from our emails.
Of course, this is all dependent on the industry of the clients you're managing email marketing for. Some of our clients are in industries where their subscribers are constantly searching for deals and promotions, so we lead with those. Others are in industries where their subscribers are more interested in tips and tricks, so we lead with those.
We were only able to determine which of these tactics worked through which industry using our signature method of TTTR: Track, Test, Tweak, Repeat. We track the data associated with each email, test out new methods, tweak the tests to try new things, and repeat!
Hope this helps!