The default knight character is a well-balanced fighter with no real weaknesses and a few nifty perks. This may not be the most exciting class to play, but if you master the Warden, you've half-mastered every other character in the game.
While the Warden has fewer powers than most heroes, their regular attacks strike a good balance between speed and damage, and their shoulderbash can used to get out of many a tight jam. Wardens are good for all roles, though they never quite become the death machines that a Peacekeeper or Kensei can be in the right hands. Still, don't underestimate the power of simple chain attacks- they may not be flash, but if you keep staking htem up, they'll get the job done.