Al-Muwaafaqaat by Imam ash-Shatibi

Probably the safest and best edition of one of the biggest Usul Fiqh's books. The Muhaqqiq has gathered 11 manuscripts!
The hadiths are authenticated and discussed in annotations, each difference between the manuscripts is detailed as well.
The edition is beautiful, 7 very large volumes. The first volume is a very thorough research on the book, its author, and the methodology of Tahqiq and the sources that were used, and the last volume is a very detailed summary (question by question) of all volumes).