Just read this article on op-shopping. It covers some of the standard ethical issues which I have confronted: the biggest for some being "did this belong to a dead person". Good to see that mainstream media is covering alternative fashion themes such as vintage and opshopping and not just regurgitating the lovely, but expensive NomD, Karen Walker, Deadly Ponies (all NZ designers) as sources of key wardrobe pieces.
Personally, I do not have a favourite item - there are too many to choose from. I do not subscribe to the "key" pieces philosophy either, I just wear what I like and what makes me feel good. Some days I make sure I am completely matchy matchy from head to toe, others I like to strike a chord in contrast. I have noticed that my work clothing style is definitely more "out there" than my colleagues, but is still acceptable in a semi-corporate education/health