The most critical phase of ERP implementation is discovery and planning. In this stage, your partner should learn about your processes, data, and ultimately the full scope of the project. This is critical to the rest of the project and how the system will be configured to your processes and data needs.
Of course, all aspects of the project are critical, but if your processes and data are not adequately understood, the rest of the project will be at risk and will not maximize your return on investment.