For me it is Napoli or Naples.
I've been to Paris, London, Rome, Milan, Florence, but this place doesn't compare. I'm not saying I love it, but this place IMO is the sterotypical Italy, with the mopeds, the dodgydriving (crossing the road is job on its own, I saw countless near accidents last night, and then you have mopeds coming up on the sidewalk as well), the pizza (only worth coming for, aside from the history sites like Pompeii), football passion (get that everywhere except Turin) and you think this place is in some 3rd world country with the bogan like attitudes with rubbish lying all over the streets and many times you are having to step over it as there is a big pile off it lying on the ground, people just throwing rubbish onthe ground (saw that outside of the football stadium - the San Paolo - a woman finished a bottle of water and just threw it onto the ground) or with a rat running round or a dead rat just lying in the middle of the piazza (square) and to which I nearly stepped on (some people were watching me, waiting for me to step on to it until I saw it, I sort of saw the funny side by laughing through gritted teeth) or stray animals, nearly every building or train has graffiti on it.
Yet I guess this what makes this place so different and unique and feels like part of the real Italy (sorry but you don't get that in rome, Colosueem or Vatican City or not) although I'm sort of happy to be going back to Rome tomorrow for a night before I go to Barcelona.
So what is the most unique place you have visited? it can be anywhere, some bogan backwater town/city, different country, or place you love a lot.