Here are a few thoughts - and I would think about this in the context of what you are hoping to accomplish. These are supposed to be leading indicators of your marketing, so I like to look at things like
- Organic Traffic Growth
- The conversion rate on the site
- Size of your MQL/SQL list that you can nurture
- Deals/Opps that marketing helped create
But, this really comes down to details about your b2b industry. i.e. deal volume, velocity, and the organizations you are selling to.