There are a number of ISP available in market and different prices and services they provide. The first thing you have to see is that the availability of the ISP in your location and the service center of that ISP. Choose according to the network speed you want and how much reliable is the ISP is according to your need.
Go to the ISP which are having less prices but may be their connection are not durable. Choose according to their future existence like I had bought an ISP and now its demands decreases at a high rate so now the services they offer are so poor. Make sure they are supplying good hardware to you as sometimes the connection is good but the hardware eats up the speed for example choose the finest modem and optic cable. Make sure that the customer dealing service is good in the ISP you choose as sometimes maybe they are not answerable to what you query is.
Choose a ISP which is reliable both in present as well as in future and the cost is affordable and choose the connection speed which may just satisfy you no need to have such a great speed an offering too much money.