Steve Smith was on the top spot since December 2015. It was only in August 2018, when Smith was serving a ban for his role in the Newlands ball-tampering scandal that Virat Kohli overtook him on the charts after reaching career-high rating points in the series against England.

from Times of India https://timesofindia.indiatimes.com/sports/cricket/news/icc-test-rankings-virat-kohli-loses-no-1-spot-to-steve-smith/articleshow/70960163.cms