Regarding the single choice – I disagree... yes, your favorite might be and SHOULD be the first choice, but you still need to choose based on the specific situation, considering the opponent, the size of the map, and so on, because it's foolish to attack a Stronghold with a Tower or the same undead, as you still need to have time to develop ;)
P.S. I always play only for those I like... for example, I don't like playing as Necropolis, the atmosphere of their castle depresses me – and that's it, no amount of persuasion will lure me into Necropolis!!!;)