1. Randy Moss, 1998
It is not even close. Randy Moss is one of the greatest receivers in NFL history and is undoubtedly the greatest player to ever be picked with the 21st overall pick. If the Chargers got someone who was half of what Randy Moss what during his prime then it would be considered a massive win for the franchise.
Selected with the 21st overall pick by the Minnesota Vikings in 1998, Moss instantly became one of the best receivers in the entire NFL. In his rookie season with the team, he was named to the Pro Bowl, was an All-Pro and finished with 1,313 yards on 69 catches. He led the entire league with 17 touchdown catches.
At the time this was the second-most receiving yards a rookie had ever finished with (which since has been surpassed by Anquan Boldin, Justin Jefferson and Ja'Marr Chase). His 17 touchdowns are still the most by a rookie receiver and have not even been close to being broken with Chase coming the closest with 13.
Let's not forget about his 2007 season with the New England Patriots, either, when he finished with 23 receiving touchdowns on a team that went 16-0 in the regular season and were a helmet catch away from finishing 19-0.
Moss ranks fourth all-time in receiving yards, 15th all-time in receptions and second in receiving touchdowns. The only receiver in NFL history that can undoubtedly say he is better is Jerry Rice. Every other receiver falls below Moss.