Hi Coldplay
I think it's fair to say that people will have different opinions on what their favourite city is, based on individual experience. Some factors that makes a place a persons' favourite would be sightseeing, the hotel they stay in, nightlife, cost, whether they have friends/family there, fun you had with certain crew - the list is endless!!
I used to be crew and I have to say that I really like BEY, CPH, BCN&FCO - these are the nightstops I loved for so many reasons.
Good luck with your application in becoming cabin crew, if and when you decide to go for it.
I can't comment on what it's like visiting SYD on a nightstop because I've never done it but I live here now so my view of the city will be different I imagine. A visitors' view maybe different to someone that lives here. Having said that, I loved Sydney when I visted from London and I still do.
Exmax