I really do like all the teams even though some are more powerful than others. I've found that it can be extremely situational, or dependant on dice rolling, as to how effective each team is. I've had games where it wouldn't matter what teams are in play because I'm rolling so bad it's apparent that I'm destined to lose that game. But I've found all the teams to have their use.
Even though they probably aren't my favorite team, how can you go wrong with "Armour of Contempt" and "Litany of Hate?" But, if I had to choose, I probably say Purple is my favorite. I love all three of their action cards.