The number one thing I hear from Dressage riders is they want their horse to be quieter in the mouth, to have less tension and resistance in the contact.
Fair, because doing so would drastically improve scores.
Often horses aren't necessarily tense or against the hand and loosing marks because it's mouth is open - caused by an uncomfortable bit or an incorrect bitting choice.
Lets go through my top picks for bitting a Dressage horse - these are in no particular order and are purely based on my opinion and experiences as a fitter.
Neue Schule Verbindend - A fantastic bit for a wide range of horses. The curve sits on a less sensitive area of the tongue and allows clear communication. It is particularly useful for horses that are prone to being heavy on the forehand or lean as well as horses that have bar sensitivity.
Fager Oscar - Oscar is a titanium bit with a front lock. It's lightweight and stable in the mouth while offering the horse a high level of comfort. It is a great bit choice for horses that don't like lots of movement or the weight of a stainless bit. Oscar is a fantastic choice for horses with a smaller tongue.
Bombers Ultra Comfy Lock Up - This is a hugely popular bit for many horses, in particular Thoroughbreds in their early stages of their retraining (Yep, they do Dressage too!) It is a simple yet stable bit allowing the horse to build its confidence in the bit and contact. Combined with the baucher cheek piece it offers poll relief for sensitive horses (although the baucher is not for every horse)
Fager Marcus - A comfortable sweet iron bit that curves nicely in the mouth to give a small amount of bar relief but also not take up a huge amount of space. Ideal for horses who aren't particularly fussy in the contact but need encouragement to take the bit forward.
Neue Schule Demi Anky - The Demi Anky is a great option for horses that don't like a mullen mouth or a double jointed bit. Its curve sits comfortably in the mouth allowing for clear communication. It is designed to apply less pressure to the lips and avoid pinching the tongue.
Naturally these bit choices always depend on the individual horses preferences, mouth conformation and way of going. If you aren't sure it is always best to contact your local bit fitter. These bits are available in a range of cheek pieces and not every cheek piece is going to suit every horse.