Proxy Statement or the DEF 14A is the most important SEC filing. Period.
New investors are often recommended to read Annual or Quarterly earnings reports or track fundamentals of a business such as Price to Earnings or Price to Sales. These metrics are important but for the most part all metrics are historical snapshots. All the publicly traded companies are familiar that both institutions as well as retail investors focus on them, which implies there exists incentive for management or owners to influence these metrics to suit investors. For short to medium term, gaming these metrics has no negative influence on the business but over long period, it has potential to not only deteriorate but sometimes wreck the business. Enron is a great example of this.
Fundamentals can be influenced to look good with accounting wizardry. However, management or ownership characteristics & their activity are relatively harder to ‘game’ for publicly traded companies. This makes Proxy Statement the most important SEC filing. It is hard to judge business quality just from the it but red flags are more easier to find out than other SEC filings.
For starters, have a look at profile of board of directors, board composition, compensation of executives and related party transactions. If these seem fine then you can go on studying more about the business from other documents. Otherwise, you need to be careful. Even if there is single thing that you find weird, be careful to invest in the company. You’ll be a lot happier when the business eventually blows up or stock price craters few quarters or years down the line.