Chilean entpreneur, Digital Project Manager and co-founder of digital agencies in LATAM, actual Growth Hacker in cacerola.cl Spanish feedbacks.
The tracking KPIs that must be your process:
1. Acquisitions tracking: If you want to know from where came your visits, Google Analytics is your tool. Is a session centric tracking, that it's really powerfull with some settings addeds.
Mixpanel and Kissmetric, both are really strong to understand who (name, email, sex, age, etc.) is doing what. It's required that your website use an register or loggin process. And need a lot of event settings to get acquisition information.
2. For granular info, a software like mailchimp can give you really strong information if it's setting well. But anyway if you use another tool, a tracking method that helps a lot it's the URL builder, here a chrome extension.
Hope this help you. And let me know if you need more information about this subject.