Perth, Sydney or Melbourne. Where's your favourite place and why?

Hi!!
I am seriously thinking of working as an aupair in Australia for a few months, but I don't know where I should pick between hose three wonderful places... I am 18, I love adventures, nature, but also busy places, parting and I want to discover life.
If you have any suggestions or even tiny little things to say please do so, I would appreciate it a lot and would be truly thankful!

Sydney is more safe I don't know about Perth, Melbourne has the best shopping Centres apprently lol but if I were you go to Sydney best place in Australia :)