SERP ranking may be the most report-friendly metric, as it's easy to analyze, visualize and explain to clients, but high rankings don't necessarily translate into site benefits. "Ranking should not be the end goal of any project," Smarty says. And because they move up and down naturally, SERP rankings may be a hard metric to keep track of.
Traffic is another metric that's easy to keep track of--and one of the easiest goals to hit. But, "traffic doesn't mean more sales or customers (the quality of traffic is what really matters)," she says.
And conversions are the ultimate metric, as they're tied directly to the client's goal of selling something (or getting subscriptions, etc.) Still, you likely won't be able to track (or influence) conversions unless you're performing the full suite of search services for said client, including SEO, PPC, usability testing, etc., she says.