I really don't think you can pick a liveaboard using only ONE of the criterion you mentioned. There are multiple things (in no particular order):
1) Itinerary: beginner, advanced, mixed, type of dives, etc.
2) Size & type of boat (steel, wood, fiberglass, etc.) How many will she hold and how many are typical for that particular time of the year? How well is the vessel maintained? What kind and how many chase boats?
3) Number of dive guides and their level of experience.
4) Amenities (private heads, other equipment/activities available). This includes food.
5) Price: not is it the cheapest, but is it the best value for the money?
6)Reputation/references. You need to check out a boat & crew's track record.
Just a few off the top of my head...
~SubMariner~