For foreigners, probably Abbottabad (population: 275,890) due to being the site of Osama Bin Laden's compound.
For Pakistanis themselves, it's a bit harder to determine, as I'm not able to find reliable population statistics for smaller settlements. However, some contenders are probably Nathia Gali, Chitral, Skardu and Ziarat. All of these towns are in the northern mountainous regions of Pakistan, which don't have as much population density as e.g. the plains of Punjab. They're also fairly popular tourist destinations for Pakistanis who want to take a break from the heat. Ziarat could be especially famous, as Muhammad Ali Jinnah (founder of Pakistan) spent some of his last days in a cottage there. It even appears on the 100-rupee note.