I've said before that I like famers walks...mine seem to grow no matter what I do.
I like pulling my sled in a 'face pull' motion too...
I know you've mentioned them often, so many benefits to this exercise, one being they promote good posture (pulling shoulders back), but i dont think that this exercise really stimulates them mechanically, there is nothing worse than seeing some dude with "slumped" shoulders.
For rear delts I will use with the fly machine (not peck deck) or do reverse cable flyes, I find I can target the rear delts much better then reverse db flys.
I think the trick is also to not go too far back else this will move the focus from rear delts to middle of the back, you be able to feel the tension be released from the rear delts when doing this.
I think you're partially right, in that finding an exercise that works the given muscle from fully stretched to fully contracted under constant tension (mechanical work) "you'll get best bang for bucks" it's the best way to stimulate growth, but i believe the "load" needs to be really heavy and I don't think those exercises (flys) cut it.
BTN barbell press for reps.
Couldnt think of antything more boring than doing delt raises/flys.
Although being an excellent "shoulder girdle" exercise, The BTN press is doing squat for your rear delts in regards to optimal development
I tried for a long time to do bent over DB raises, but I just can't enjoy them. I tend to do face pulls or standing rear cable flyes. I was enjoying doing a tweaked version of DB rows for a while there so that it hit the rear delts. I suspect my answer is single joint..
Rows are the key, chest supported with heavy weight with a grip just wider than shoulder width allows one to really fatigue those shoulders.
I rely on back work and upright rows for my rear delts.
"back work" is the salient point here.
All forms of pulling, those big multi-joint movements biuld big rear delts.
My faves, in no order and the one's i do throughout the week and in the last 20 years solid:
chest supported row
machine pullover
deadlifts
SLDL
pull-downs
chin-ups
Farmer's