Subjects you are interested in:
Everyone says this but it's so true: do subjects which you enjoy! You are going to be doing these subjects for two full years. You need to be doing subjects that will keep you motivated to study to ensure success. Also people tend to do better at subjects which they enjoy because naturally they will put more time into it. Even if the subjects you are interested in scale down, it's still worth doing them because you could end up with a high mark even with the mark down.
I personally did mostly humanities based subjects which scaled down but I really enjoyed doing them which allowed me to do very well and way better than I would've done if I were to have done all maths and sciences. If you aren't interested in a subject you probably won't do well in it because you won't study as hard. Personally, my top subject was the one that I enjoyed the most and my bottom subject was the one which I really did not like. Unless you need a certain subject as a prerequisite, don't do any subject that you don't enjoy.
Look at what subjects you're school typically does well in:
If you are aiming for a high ATAR, you may find it helpful to do subjects which your school typically does well in. A lot of schools are known to do well in certain subjects and some schools actually post their average study scores for each subject online. I'd check out what subjects your school excels in and if you are interested in the subjects, then do them. Looking at your school's average marks for certain subjects can be particularly helpful if you have too many subjects that you want to do. It's an easy way to decide which subjects to rule out or which ones you will most likely do better in.
Remember though, if your school does not typically do well in a certain subject, it does not mean that you won't do well and vice versa! Take your school's average study scores with a grain of salt.