In my honest opinion selecting a 16 bit ADC is a trivial process once you have decided on the type to be used - SAR / DSI / Sigma-Delta....

And deciding on the type is a part of a major issue pertaining to accuracy and precision that you want to achive finally. This particular thing is HIGHLY dependant on your PCB layout, power supply, grounding considerations and signal handling discipline. And since we are talking ADC, there must be an analog sensor ( source ) - what level does it provide ? what signal conditioning is done ? who is doing it - you or the supplier of the sensor ?

Planning to get 16 bit resolution is one thing but actually getting it is something else. This has been discussed many times on this forum. Check them out.

Raghu