The Good Friday Agreement

 On the 10th April 1998, the 30 years troubles that affected the island of Ireland (both north and south) and the UK had come to an end, by a single piece of legislation. This piece of legislation helped to cement the fact that Northern Ireland remained part of the United Kingdom; however, it allowed the Irish minority to believe that the north of Ireland was a part of Ireland if they so desire. Citizens of Northern Ireland can now choose to be Irish or British (or both) and the lack of any border (which was crucial to the Irish backstop) was essential in creating the perception of a United Ireland to Nationalists/Catholics. More importantly, the fact that a new government was established where Nationalists and Unionists had to share power - they had to work together and peacefully. I personally think the GFA 1998 (which Brexit puts at risk especially with the border in the Irish Sea) is the best piece of legislation the UK (with help from the EU and the US) has produced.