A tricky question to answer! Ultimately it needs to be whatever is going to best resonate and engage your audience. This is likely to differ for brands and organisations. Testing your messaging - whether this is headlines on email or page titles on webpages - means you can start to understand more about your audience. Devise a list of hypothesis to test, run the tests and analyse. And it's worth considering 'human truths' when you're thinking about what could grab attention.