There is no "best" PIM software, as the needs of each user vary greatly. However, there are a few well-rated options that may fit your needs.
-EfficientPIM: This software is designed to be easy to use and intuitive, with a focus on efficiency. It offers features like task management, contact management, diary, and more.
-David Allen's Getting Things Done: This popular productivity method has its own software to help you implement it. The software includes features like to-do lists, goal setting, and tracking your progress.
-Toodledo: This software includes features like to-do lists, task management, and notes. It also integrates with a variety of other software and services, making it a versatile option.