Calc is the spreadsheet program you've always needed. Newcomers find it intuitive and easy to learn. Professional data miners and number crunchers will appreciate the comprehensive range of advanced functions. Wizards can guide you through choosing and using a comprehensive range of advanced spreadsheet functions. Or you can download templates from the LibreOffice template repository, for ready-made spreadsheet solutions.
LibreOffice Still is the stable version that has undergone more testing (over a longer time).
It is usually recommended for more conservative use.