Most of the best spells have already been stated, but I thought I'd add a couple of other ones that have so far been ignored.
Calm Rage - Very important in boss fights to keep your Mana high since you aren't allowed to use the Dragon.
Awaken Dragon - Great spell for obvious reasons. I often use it to open two Treasure Chests in a round.
Mass Battle Cry - A surprisingly useful spell for keeping losses down against enemy high-intellect Heroes. On the last turn, cast this, do your Resurrections, and finish off his units before the opponent gets to cast. It won't be used too often, but it's a good spell to have in your arsenal nonetheless.