What group of people did Hitler fear the most, and why? The "why" question being the important one.

The group of people that Hitler feared the most was the Jews.  He feared and hated the Jews because his ideology held that the Jews would be able to destroy German culture if they were allowed to remain in Germany.

Hitler’s ideology held that there were three general types of people. Some people, the Aryan race, were the ones who could create culture.  He believed that every truly advanced and civilized culture in the world had been created by Aryans.  A second group of people was incapable of creating culture, but could keep a culture going if they were given that culture.  The third group of people were destroyers of culture.  They would ruin any culture that they were a part of. The Jews were the most important culture-destroying group in Hitler’s mind, though the Gypsies (Roma) were also in this category.  Hitler believed that Jews inevitably destroyed any culture in which they lived.

Because there were a relatively large number of Jews in Germany, Hitler feared that German culture would be destroyed.  He feared that the Jews would destroy German culture because Jews were naturally people who destroyed the cultures they lived in. Therefore, he feared them and wanted to remove them from Germany by one means or another.