I like Karachi because of its big city feel, it being near the coast, mixed cultures, some of its very Anglo buildings and the fact that its not too hot in the summer. Downside is the bad water, can be very built up in some areas, political problems.
Islamabad its green, well spaced out, peaceful, near the Margalla's and the beautiful North, good climate even in summer, but expensive!
Quetta - I like the geography there, wild and desolate and could keep me interested for a a while although itcould get monotonous. Its a bit far from most urban/built up areas although the city has a nice mix of tribal people ad languages.
Lahore big, peaceful, historical, congested in some areas, green in some areas. A busy and lively city more character than others that for sure.
Multan - not too far from other urban area and in a central location with North Punjab, Sindh and Balochistan not that far away. Its a big city, exotic, has history but lacks green spaces in the city and less cosmpolitan. Its too hot in summer.
Peshawar - near the north and Urban Punjab is in easy reach. Big city, lots of character, not very green, not too cosmopolitan.
Abbottabad - not very big but very green and very close tothe North and Punjab. Plenty of placesto visit in the region, gren city but small. Weather god through summer.
The three I would chose would be Islamabad, karachi and Abbottabad!