I don’t have any one best exercise for training the adductors. It depends on my client’s assessment and how other areas of the body also move. Alignment, balance and stability has a lot to do with which exercises I choose for each individual client. I also keep the clients current activity choices and goals in mind when choosing exercises.
That being said, any exercise that has your leg moving toward the mid-line of your body will train the adductors: Side Laying leg lifts (bottom leg), side laying leg circles (bottom leg as the mover), Standing leg lifts with band, side to side walk with band tied around legs, etc. These are just a few that I have used in the past.