To see Zipf's law, one must take ALL (or a large amount of) words rather than rare words. It is like to see Zipf's law in cities, one must take ALL cities (or settlements to be more precise) rather than largest cities. If you take a part (say, the largest cities) for the observation, the part must be corrected one rather than an arbitrary one; see the following illustration.

Jiang B., Yin J. and Liu Q. (2014, accepted), Zipf’s Law for all the natural cities around the world, International Journal of Geographical Information Science, xx(x), xx-xx, Preprint: http://arxiv.org/abs/1402.2965