I like 1am-2am, I specifically used to like going running at this time. Or walking home early morning, 5am, before most people are up and out. There's this nice hush over everything. Just a milk float (going back in time there) and the world starting to stir.
I like when the place is asleep, I used to love going into the City of London after a certain time or on a weekend when it seemed completely empty. It could feel like you were the only person there.