With 50% market share in just one month 1803 (aka April 2018 Update) is the fastest spreading Windows 10 update by far. It took Fall Creators Update twice as long to cross the 50% line, and Creators Update was under 50% for about 3 months.

Note: we don't have the data for the initial rollout of the first update to Windows 10 (1511, also known as Threshold 2). So it's a straight line from 0 to the first data point we have on August 22nd, 2016.