There is no best, all will work fine in any content, it's really more what you prefer and how new you are. WHM is easiest to learn if you're completely new like I was (had never even played a MMO before). SCH you have your pet to deal with and AST the cards. WHM is easier to learn healing on I find since it's more basic. It's not that it's super easy or anything like that, it's just all your tools are focused on healing, either area healing, healing over time, direct healing, instant healing etc, where AST has buffs for the party and SCH has shields and the pet.

Put it this way, you'll never get to a point where you're like "Damn I can't do this new content because I chose AST, I wish I had done WHM now."