I don’t know about “best” but I work the wide grip pull-downs and seated cable rows to strengthen my lower trapizius, and the client this is most helpful with could be ones suffering from neck pain due to tight upper trapezius tension or those with rounded shoulders (most people I’ve worked with have weak lower traps, fit and unfit alike). You could also add prone Y raises to this list.
I use an exercise that I call ‘Reverse Shrugs’. For guys who usually have longer arm length, I have them set on a bench with arm straight and slightly behind their bodies and instruct them to push the shoulder blades down. Since I have a dual axis cable tower, I do a similar exercise with individual loops hanging off the cable attachment. There the clients sits on a ball of bench and gets the same cue.
This may work but try this which is more anatomical and biomechanically correct.
remember the low traps originate from T6 to T12 and perform scapular depression/retraction.
Have the client with their knees slightly bent and have them reach above their chest level with both palms facing up and pulling down a bar or cable that has a bar at a 45 degree angle to the chest as if they are elbowing someone behind them.
This targets the low traps and is a great exercise with clients for a history of shoulder impingement, RTC repair, bursitis, acromioplasty which all need to unload the anterior shoulder and assist with restoring the upward rotation of the scapula to glide on the thorax.
This upward rotation gliding motion occurs when you strengthen both the Serratus anterior and low trapezius together biomechanically.