Actually berserkers aren't all that rare, so long as you are playing Viking era. The Picts generally seem to get some. Jom Vikings, however, I have never seen unless I play the Vikings and make them myself. I don't think there really is a rarest unit. As Jxrc points out, it all depends on the era you pick and the faction you play. You could make Lancers as common as mushrooms if you wanted to play that way. Leave it to the AI, and you'll probably not see any.