I personally prefer Perth, but that's because I was born here and grew up here, so it's my home
I've been to the other cities,and out of those, I enjoyed Melbourne the most, followed by Sydney. Melbourne is good to experience all weather in a day
I remember Christmas 2000 - it was 40 degrees in the morning followed by a thunderstorm in the afternoon! exciting.
Perth, compared to the other cities, is very small, very quiet. The city parking is a lot cheaper, there are a lot less people. The traffic is less, everything is less. If you like busy-ness, then I'd recommend Melbourne