In my experience German Embassy has the quickest response time .. two weeks
you can get the requirements from here
Deutsches Generalkonsulat Karachi - Visit Visa
The requirements of most other schengen member countries are similar
Travel Insurance for the duration of stay ,
invitation letter from someone in the host country(I am not 100% sure would that also be a requirement for a tourist visa) ,
return ticket itinerary ,
hotel booking for the duration of stay (or if you are staying with someone in germany; their details),
6 months bank statements,
source of funds from wher you will bear your expenses in the schengen member country; for business VISA’s it usually a covering letter from your company, if you are visiting a relative it could be a sponsership letter from them.
If you are submitting any letters on behalf of someone in the schengen member country.., they will require you to fax the letters to the embassy ..,
Italy i think has a visa turn around time of 4 weeks,
I have some friends who get Swiss VISA within 3 business days but then their company has a special relationship with the Swiss Embassy.. , so i would recommend apply through germany.
Also one more thing if you get a VISA from one schengen country but would want to travel to another plan your trip in such a way that your entry & exit should be through that country or else that country would never give you a VISA again.
Hope this helps