As far as I am aware, the highest scaling subject from the subjects that you are contemplating is Science Extension. The other three subjects are not considered to be high-scaling. However, scaling should not be a primary consideration when selecting subjects. Instead, you should consider a subject that you are interested in (or the subject that you find least uninteresting of the five subjects that you have listed) as well as the one in which you believe you can perform at the highest standard. In doing so, it may be helpful to recall your performance in some of your Stage 5 subjects (for instance, Science and Geography).
With that being said, it is important to note, if you are not already aware, that Science Extension is meant to be taken in year 12 and has a value of 1 unit, in addition to the fact that you will need to keep at least one of Physics or Chemistry to be eligible to take Science Extension.
Additionally, it would also be a good idea to consider the degree(s) that you intend to study after you finish your HSC. From your current subject combination, you seem to be interested in a degree relevant to engineering, science or medicine. If that is the case, you would essentially be taking all subjects that would be needed for a comfortable start in such degrees, except Biology, which you may wish to consider taking if it is relevant to what you wish to study beyond the HSC (Biology is treated as assumed knowledge for medicine). Otherwise, you are free to choose any subject that you feel interested in and believe you can perform well in.
I hope this helps!