Start from the decision
A report exists to support a decision: spend more, spend less, change something, carry on. Any metric that can't change a decision is background noise.
The three that usually matter
Cost per qualified enquiry. Pipeline or revenue attributable to marketing. Rate of change on both.
Everything else — impressions, clicks, rankings, engagement — belongs in the appendix as explanation, not on page one as achievement.
Say what you'd change
A report without a recommendation is homework for the reader. Every monthly report should end with what we're changing and why.