Umme aafia,
I-14, I-15 and I-16 are Zone II low cost sectors and are good for long term investment of about 5 years or more.
G-13 is a fully developed zone I sector and is much more expensive than I-14, I-15 and I-16. You should buy in G-13 for living point of view and not for investment point of view because this sector has already matured and sudden jump in prices is not expected.
G-9 and G-10 sectors are considered the 'real' Islamabad sectors (but not posh Islamabad sectors) and land there is even more expensive than g-13 because of proximity with main Islamabad. But houses in g-9 and g-10 are 30+ years old. It is a good idea to buy in g-9 and g-10 if you can get a reasonable house which is close to the price of the land. Why should you pay too much for a structure that is old. You can then renovate the house with further investment and hopefully make a quick profit on sale of renovated house.
But for G-9 and G-10 you need to do the purchase very wisely and consider the house from many angles (i.e. Location whether close to market or main road or good school; whether the street is wide and what kind of houses are in the neighbourhood, where is the nearest green belt etc; age of house; can it be renovated and what will be the cost etc. etc.). Then this is good for investment because you can make quick return and not wait.
Now for investment in plots, i would advise you to go for zone ii societies like f-15 (non-possession); e-16; b-17 (multi garden). In zone I, G-14/3 is a good option for return on investment expected in 2-3 years. For long term investment in zone I, it is a good idea to invest in e-12 (but be careful that the file should not be fake - check with someone in CDA).
Best of luck!