There's no bad option when it comes to playing Justicar Trueheart, since it's a neutral minion that will buff any class you want to play.
His battlecry, "replace your starting Hero Power with a better one," will upgrade the hero power of whatever class you're playing. For example, Paladins will summon 2 1/1 Squires instead of 1, Hunters will do 4 damage to the enemy hero, etc.
Justicar Trueheart is by far one of the best legendaries of the expansion because of the immediate return you see with any class. He's very overpowered in any style deck.