You can't really ask that. Different schools do different topics in Prelim. It's not like Chem or Bio or Phys where there's a set topic for Prelim and HSC. For example, our school did Perms/Combs as one of the very first 3U topics, but most schools don't do them till HSC year.
Otherwise, the hardest topics are probably Perms/Combs because most people don't have a solid understanding of it, Physical Apps of Calculus (or maybe just for me cos I hate physics stuff), and Binomial theorem (because it intimidates so many, but its actually not that bad)