in Karachi, there arent very many good ice cream parlours as there are in Lahore...Gelato, Movenpick, Mars, Dipshop etc. Havng said that, you should try out the traditional kulfi faloodas at Sohni Sweets, Sunshine Sweets or a tall glass of the rose flavoured falooda at Balouch Ices or Flamingo in Clifton, New Town Masjid area or any good juice corner.
I'm sure Shezan still serves their "Shezan Surprise" havnt had that in ages! And the aslee doodh ki Kaybees ki icecream...you should see the rush in the evenings and on weekends. Its like there's not going to be any more ice cream available anymore.