Your ranks appear to be optimal across your subjects, especially in Mathematics Extension 1 and Biology. As such, I would question your decision to drop a subject if you are not experiencing any performance or time management issues. Your favourable results across your subjects would indicate that you have an effective study strategy and that you are able to manage the workload from each subject well. Of course, this may also partially be due to you attending a lower-ranked school and/or your school setting easier assessment tasks/exams (if applicable).
However, if you wish to drop a subject despite this, then from the information that you have provided, Studies of Religion II would likely be the subject that you should drop. Here, it would be good to gain an understanding of your school cohort's performance in recent years, which can be through your school's rank and/or the number of band 6s/E4s achieved at your school. This would allow you to compare the typical performance of your school in SOR II with another subject where you are not ranked first, like Chemistry. At that point, if you still wish to drop a subject, you will be able to do so knowing that you are making an informed decision.
I hope this helps!
