Who was the manager of Derby County when they achieved their biggest win?

The manager of Derby County when they achieved their biggest win was Frank Lampard. He was appointed as the club's manager in May 2018, ahead of the 2018–19 season, and achieved immediate success with the club. Under his leadership, Derby reached the Championship play-off final and won their biggest ever victory in the final against Aston Villa, with a score of 2–1. He guided the team to their first play-off final in 12 years, which ended in a victory for the Rams.

The win was a huge achievement for Lampard and the club, with the victory seeing them promoted to the Premier League for the first time since 2008. Not only was it a personal success for Lampard, it marked a major milestone for Derby County, who had been struggling in the lower divisions prior to his arrival.

Lampard's success with Derby continued into the 2019–20 season, as the team achieved their highest ever points total in the Premier League, finishing ninth in the table. Lampard left the club in January 2021 after two and a half years in charge, with the club in fourth place in the Championship.