I've played through BOF I, III and IV.

I think III is the best. I like the whole childhood/adult transitions. I also liked the way the villains were motivated by misguided altruism instead of unprincipled evil. The 'SimFairy' subquest was addictive.

In BOF I though, the relatively minor characters of Cerl and Alan stuck with me in a way that the main characters did not. I found their story poignant.