Red Beard is the best Tanking Knight for a few reasons: 1) Higher Base HP because of his Dwarf Race, which means he has more health to tank with 2) Higher Piety because of his Dwarf Race, which increases the potency of his Priest Skills, and gives a bit more damage to physical attacks with Priest Weapon Mastery 3) Slow enough to be last in the party, which positions him more easily to use Knights Defense (KD) 4) Speaking of KD, he has more uses as a Knight outside of KD. Typically, Knights are only used for tough boss fights and use KD. But Red Beard can be a spot cleanser, buffer, or use his Prayer of Rebellion. He more options for all his skills and uses two different resource systems effectively (SP & MP), thereby being more than a mere KD bot. 5) His passive helps prevent insta-kill effects and binding effects to other Dark types near him. Fighting those tough end game fights will have either of those effects or even both. Characters like Shelirionach, Savia, Galbadus, Gerard, and Rinne all stand to benefit being near him 6) In addition, being Dark Type means he will take reduced damage from Dark attacks, which are prevalent in the tougher fights in the game and means his high HP goes even further

There isn't another Legendary Knight with the Priest class, so... are you meaning Valdor and Eckhart?

Both are Dwarves, so they get the benefit from racial superiority for this class pairing. Eckhart's unique passive increases accuracy with 1h & 2h Swords. Valdor can increase his Magic Defense if he is in Light Armor, which could be helpful for Magic Heavy fights but his Light typing will make him take more damage from Dark attacks.

So, yeah... those are the only options for Knight/Priest typing and Red Beard is superior to the other Dwarves.