well most of the comm will say whm is the easiest and most boring heal to play... for me (i own all three at max gear atm) sch is the easiest one to play in combination of support and dps at the same time. however whm comes with the strongest burst dmg, sch provides best dps for longterm fights but defintly lacks with burst aoa dmg. ast is for sure the "newest" or let's say most modern cls of the trio. synastry, cards, choice of stance, dots, hots, burst dmg and self buffs are way more complex than the other two.
for me – everybody who enjoy healing at all should max them all, they are all nice to play =)
as a sidenote if you a familiar with all three healers, you are ready for every raid or trial grp-constellation. for me there is no "best" healer, they all shine or whine in certain situations ; )