as he posted it in the 15-19 bracket then obviously that's the bracket the wants to play in.
all classes are viable if you play them correctly, the better ones are hunter, priest, paladin, rogue and mage while warriors warlocks shamans and druids are underdogs.
All classes worth playing, depend of you, if you like being OP and not having to care about the mistakes you make go hunter, priest or paladin, if you want to still have some challenge you can go druid, rogue or mage and if your a gimped class fan go lock, shaman or warrior. All classes can be great if played well, good luck with your new tinker