Continuity with the ad
The single biggest win and the most frequently missed. If the ad promises something in a particular way, the page heading has to continue exactly there, in the same words. Any difference in phrasing produces hesitation, and hesitation on paid traffic translates directly into lost money.
In practice that means several page variants for several ad groups, not one for all of them. We build them on the same skeleton, so the additional cost is small.