Trantor was one of the first, wasn't it?
For me there's also William Gibson's Japanese Chiba City, which strangely always reminds me of Blade Runner's Los Angeles. (That one really has set the visual standard for so many, hasn't it?)
I think my favourite though has to be Arthur C Clarke's Diaspar and Lys from Against the Fall of Night/The City and the Stars. They were two of the first I encountered, before Trantor, I think. Two cities together - one belowground and deserted, the other a modern dome city so beloved of SF.
Guess that also means I should mention James Blish's New York from Cities in Flight too, with the iconic Analog cover: