3. OL Justin Pugh/Weston Richburg
These two players may not have been elite by any means. However, at the time the Giants let them go and even after that, they were two of the most consistently solid players at their positions. However, instead of having a better all-around offensive line, Dave Gettleman instead opted to let both of these players walk in order to throw all of his eggs into the Nate Solder basket.
At the time, this move wasn't seen as terrible, as Solder was one of the best tackles in the NFL. However, looking back on it, this move would prove to be disastrous. Not only was Solder terrible in almost every season he spent with the Giants, but the rest of the line was just as bad since Pugh and Richburg were gone.
While Richburg would retire early due to injuries in 2019, it still would have been nice to have kept him around for a little while longer. Even if they just kept Pugh, the team could have been much better. Since leaving the Giants, Pugh has been a rather important member of the Cardinals' offense, helping protect Kyler Murray.
The Giants had finally found two solid offensive linemen to help the rebuild process and they were thrown away for nothing so Gettleman could make a stupid signing. Since this, the Giants still haven't recovered and the guard and center positions have been a turnstile each year since 2017.