Kinda depend on what level you will be playing at.
As of today i would say (this can change though with 7.1.5 and other things):
- Just playing around with friends then you can play whatever you feel like.
- If you pug alot I would go, Mage, Hunter, DH, Monk. Pugs often have a reference to get good classes, even for lower m+. Speaking as a 880+ feral druid that sometimes get declined to 2-6 keys just because of class/specc. Even though I would prob do the most dmg over the whole dungeon.
- If you are pushing higher m+ with a fixed group. Talk to your group and see what you want. I.E. aoe stun/range/melee/CR/TW and so on. But yeah usually Mage/Hunter/DH/Monk is strong.
Our group is running druid bear, resto druid, feral (me), ret pally, mage. Not really optimal but still done +10 in time and clearing +12 each week for weekly chest pretty easy. Gonna try +15 in time when affixes arent aids (EU has raging/nerotic/fortified this week.........).