You can choose a race once and play as it. I never choose a race based on the opponent. I only play those that I like. Right now, it's the undead.

Regarding the one-time choice - I disagree... yes, your favorite may and SHOULD be just one, but you should still choose based on the specific situation, considering the opponent, map size, and so on, because it's foolish to venture onto an S-map with a Tower or the same undead, since you actually need to have time to develop ;)
P.S. I also ALWAYS play only those that I like... for example, I don't like playing as necros; the atmosphere of their castle depresses me - and that's it, no matter what you offer, you won't lure me into the Necropolis!!!;)