I think the question has to be not "what bit is best for this breed?" but rather "what bit is best for this horse in this discipline?"
We should start with the proposition that the bit is a communication device, not a brake system or a head setting system or anything else. It's there to allow the rider to effectively communicate with the horse.
There's an excellent book by William Langdon entitled Bits and Bitting Manual
. I think you can find it at Amazon. Libertyville Saddle Shop also had it at one point. Google, again, might be your friend.
The book is valuable as it explains in great detail the different types of bits, their actions, some of the consequences of those actions. There is a companion book by Langdon entitled Training With Bits
. I'm not as comfortable with all aspects of this one, but it's also pretty good.
I'm not a "snaffle bit Nazi" but IMO the simple snaffle is the "default" bit. Once you have a horse going well in this bit you can look to use more powerful bits to improve the communication between horse and rider.
Some disciplines require bit types by rule and some disciplines require bit types because that's what's going to work in the job to be done.
Remember, too, that bit selection means making two intelligent choices: cheek piece (curb or snaffle) and mouthpiece (broken, French Link, mullenmouth, etc.).
I don't think there is a "school" answer to this question, beyond the above.