Reddit user Working-Army8590 did a crit of over 1 billion against Uber Lilith whilst playing a Mendeln Summoner Necromancer build.
Mendeln is an endgame Necromancer build built around 
Ring of Mendeln. It’s literally the best Necromancer build you can possibly play in the ongoing Season of the Malignant.
As you can see in the video below, the build is powerful enabling you to deal 1 billion crits against Uber Lilith.