For all those who are saying "Karachi" understand that Karachi is a HUGE city! It is among WORLDS 10 LARGEST cities so you cannot categorize it as being dirty or clean judging on the part you visited. While there are slums like any other large city, there are posh areas as well. All in all, it might be really dirty but the dirtiest in Pakistan? I do not think so!! (and NO i am not from Karachi)
As for the dirtiest, in my opinion, it have to be Hyderabad. If you travel to Hyderabad by train there is NO WAY you can miss the station as the staunch smell (if you happen to be traveling with open windows or are out in the car for some "awaragardi") will welcome you to Karachi. The most common reaction will be that you will look out of the window to see what it is and trust me, for next 10-15 Km the train goes through the dumps with no station in sight. Like, after smell you think you are near the city now but it is still 10-12 km of dirt and dumb you have to go through.
Similarly go to old city, their main parks etc, everything is just so so dirty!
And
I say this all after being in Gujranwala etc!