The highest ROI processes should be the first to be automated. The best processes to automate first are high in volume and require a lot of manual work. They are processes that will create a big impact on a business if consistently performed faster and more accurately.
Most companies start by focusing on tasks that are transactional and directly impact cash flow. Those will affect a business immediately with the biggest tangible impact.
After some time companies also experience the intangible benefits of automation, such as an improved customer experience, less days to collect receivables, higher employee morale.
Eliminating redundant and repetitive work has been proven to increase levels of engagement/productivity.