It is my understanding that aMule (and every other ed2k program) selects the chunk to download randomly.
As I see it, it would be a good idea to first download the chunks that the fewest users have. This way will a rare chunk be made more available, and the chance for the chunk to be lost forever minimized.
I think such a feature would gain the entire community, but I am no expert on the subject. What does the experts have to say?