Perimeter defense is an essential part of winning basketball. The key elements to a great defense are simple, seal off the perimeter and challenge jump shots. Opponents have a harder time sinking a jumper from 15 feet out with a hand in their face than an open dunk.
Therefore there are two elements to a good perimeter defender. First, there's the element of shutting down the defender. Second, there's the element of challenging the jump shots.
Measuring success in these areas is another task altogether. There are some traditional stats, like steals and blocks, which do absolutely nothing to reveal this information.
There is offensive rating, which can be an element of team play.
There is Opponent's Player Efficiency Rating or oPER which measures the PER of the opponent while the player is on the court, but that has flaws too. For instance, it counts assists against a defender. If a player stops his opponent's penetration and that opponent passes out to another player who makes a shot, the player's good defense (stopping the opponent) actually counts against him.
There are also Synergy numbers which are tracked according to the primary defender on the play. These can also be affected by help defense. However, they are better than the general defensive rating.
Synergy numbers can break down play types as well. Isolation defense is less dependent on help defense, so there's a big difference between isolation numbers and overall numbers that we can find out about if the player is helping the team or the the team is helping the player.
Also, some of the best defenders guard the opponents' best players, so that's going to inflate their oPER for two reasons. First, their opponent is going to have a higher usage rate. Second, their opponent is going to be a better player.
Giving up 20 points to Kevin Durant on 18 attempts is a far better defensive performance than giving up 16 points to James Johnson on 12 attempts. If you just look at the overall number you won't get that.
There is no magic formula for defense and while each measuring stick has its own flaws, they also have their strengths. When we look at the composite we can get an idea of what kind of defender a player is.
There's also the old fashioned eyeball test. I agree that you can't just go by the numbers because sometimes the stats can lie. Then again, so can our eyes.
That said, here are the best perimeter defenders on the market along with relevant stats and observations.