Every horse is different. In my experience, however, I always start my horses in a loose ring snaffle.
Maverick was always ridden in a pelham fr more leverage at a young age (6 years.) I switched him to a simple jointed loose ring snaffle and he responds entirely different. He engages more and reaches for the bit. He has so much more flexion. It was a perfect transition.
Ripley was ridden in a corkscrew D-Ring snaffle. I switched him to a loose ring snaffle. Different horse.
Lily, a horse I used to ride, was ridden in a corkscrew snaffle and a pelham for jumping. I switched her to a loose ring snaffle. Again, perfect change.
So in my experience, I am a big fan of loose ring snaffles with thinner bars and a comortable curve. My horses are almost always foaming after a ride (:
distances are like men. never grab the first one you see; it's never the best one, more will come along.