There are lots of things to look at.
Generally you look at input/output voltage/current ranges to see if the meet your needs. Then you look at resolution, most of the time higher is better but sometimes you don't need a ton of resolution, if you only need to read 0-10 volts in 1 volt increments you only need a 4 bit ADC, but if you need to measure 0-10 volts in 1 millivolt increments then you need a 14 bit ADC.
There is also the problem of speed, the higher the resolution the faster it is going to need to read. (0-10 volts over 1 second = 62.5 milliseconds per bit for a 4 bit, or 1 second = 61 microseconds (.061 milliseconds) per bit for a 14 bit).
It may help to know what the application is.