White Mages are the top healers for two reasons.

Emp +2 pants
Curaga III /IV

Anyone who says differently isn't being honest. WHM/SCH with those pants can pump out stupidly cheap cures and if it's three or more people needing healed then absolutely nothings beats Curaga III /IV. One Curaga IV is faster and cheaper then three Cure IV / V's and Curaga III is just stupidly efficient for both it's MP cost and casting time.

About the only times where a SCH can heal anywhere near as good as WHM is when you have a single DD on something and the SCH's only job is to cycle Cure III / IVs with a good Regen effect.