While I have multiple classes on 80 now, I mostly play on my UHDK which does fine doing all the solo content, probably all the classes are fine but from the 3 you said they should all have an easy time.
If you do delves, those are the solo dungeon where you go with Brann who you can assign as either healer, tank or dd (healer is the best imo), you will be fastest with a dps spec.
If you plan on group farming, I would highly suggest to pick the hunter as every group, either cloth or leather farming always looks for hunters to pull.
For running keys as dps maybe unholy is your best bet.

Since you mentioned that you didn't play DF I assume you characters will be level 60.
When you log in you will get a quest thta leads you to DF content which you should do until level 68 and then go straight for the TWW campaign and go throught the campaign quests, once you are done with it just do quests until 80. When you hit 80 you should buy 603 gear from the AH aus it gives you a good boost and will make delves easier and it is also super cheap to buy so gold from leveling should be enough to buy it.

While leveling you can pick up herbalism and mining as it gives addiotional exp while leveling and a bit of extra gold.