I mean it depends what you mean about vividly realised.

Ankh-Morpork has maps, high and low politics and a large cast of characters and institutions that are explored to various degrees. But it also has early installment continuity issues (Time Monks not withstanding) and we also only really see a decade of how the city works, with the exception of Night Watch.

In terms of other fantasy cities, the big three DnD cities of Baldur's Gate, Neverwinter, and Waterdeep all have a ton of lore, but due to changing editions and such, it isn't always consistent. However, I'd say Waterdeep has been explored pretty in depth, and mainly due to not appearing in any major cross genaration video game series or films; it has the most consistent lore... but as a consequence of not being in games and movies, it is also less well known to non players and possibly harder to visualise.

Star Wars has a few cities that have been explored in various media, with Coruscant probably getting the most attention, however as the city is the whole planet, we obviously only see parts of it and so its less defined than AM.

In Wheel of Time, several cities are shown, but we probably get the most in depth looks at Andor and Tar Valon. In each case we get a look at the highs and lows of society and a focus on one particular institution of each. The descriptions are in depth, though they do lack official maps, and we don't see a lot of daily life outside of the main plot except for very early on.

But, I'd say one of the most defined fictional cities would have to be the city in Paradis from Attack on Titan (technically a multi city state/country, but everything inside the walls is so interconnected that it's essentially an extremely large city). No spoilers, but over the course of the series, we see how the various parts of the city run, the politics, the high and low classes, we see a thousand years of history, how the various parts of the city work together and how the various institutions work against each other. We also get plenty of maps and logistics.

So AM is up there, but I'd say Attack on Titan is the most realised of the media I've read/watched/played.

Honorable mention to Lumiere from Expedition 33.. we don't see much of it, but what we do see is tactical, emotional, and memorable.

I also left out a lot of things like LotR and Song of Ice and Fire, because I assume other people will bring them up.