Brother always go for AC sleeper, its the most luxurious class hence the most expensive. Extra wide berths and comfy too plus washroom would be in your cabin. Cabins can be of 02 people and 04 people. Now only Karachi Express, Shah Hussain, Tezgam & Khyber Mail have them. Written the name in accordance to priority with Karachi Exp on Top and Khyber on bottom. But the issue is these get booked much much in advance so if you see availibility of AC Sleeper in any of these train, go for it immediately.
After come Business Class, it is comparatively same size cabin but with 06 berths for 06 people with washrooms on the ends of the bogey. This is the next best option and is usually available to book unlike AC Sleeper. Priority wise go for Green Line, Karakoram Express, Pak Business Express, Karachi Express, Shah Hussain, Shalimar, Tezgam and last Khyber Mail.
Next comes AC Standard, these are old bogeys with lowest AC Class fares. These are open as in no cabins, all 70 people will be sharing whole bogey.
Shalimar used to have Parlor class, wide seats not berths to travel in sitting position. Recently government has abolished this class so its nor more available.
Economy is you know no AC. This is more suitable for people on tight budget. For me down side is dust gets in the whole bogey of which I am allergic.