If you want it to be legal for Dressage you wouldn't be able to use anything with leverage, or a Waterford. It has to be a smooth snaffle. I would suggest a Full Cheek Snaffle with a French link, and use bit loops for schooling. The loops do add a little leverage, but because of this you'd have to take them off for shows. Hope this helps!!!
I just researched the Waterford, I think it would be a great option for my ottb. I think I'm going to give it a try. Thank you for the suggestion!
Waterford or slow twist d-ring. Leverage would be cimberwick but it's a pretty big jump in severity
Also something that is legal in training level dressage and hunter