Two of the best statistics to evaluate how well a defense is playing are explosive plays and turnovers.
By preventing huge chunk plays, a defense forces the offense to string together long drives in order to score. By making them run more plays, you increase the likelihood of the offense making a mistake.
Generating turnovers is important for obvious reasons. It gives your offense more opportunities to score and allows you to not have to defend as many plays.
If a defense does a good job of limiting opposing offenses’ big plays and produces big plays of its own, the other statistics used to measure a defense will usually be good as well.