What is the most important financial statement in the annual report, that investors should look at?

I would say it is really difficult to pick one or two. For one, you must look at various metrics to get a better understanding of the company and that is why people even bother looking at annual report in the first place. Metrics like Revenue, EPS, forward growth guidance are usually reported firsthand in quarterly releases already.

In fact for US listed companies there are data crawlers that instantly extract the moment quarterly financial statements are out and to immediately prices in news releases to their DCF valuations. Therefore there is often no point trying to evaluate if stocks are correctly priced from a fundamental quantitative perspective.

I would say it is more important to look at things outside of quarterly statements.

While many people tend to focus on overarching metrics like P/B, EV/EBITDA and Sales/Revenue (esp for growth stocks). I would focus on yearly data aggregation through things such as revenue breakdown by geographical or product segment, in depth look at cashflow management of company and their exsiting ST and LT debt management.