Do you mind sharing your goals in wow such as M+15, wPvp, gold farming, transmog? That should help the community to give you some advices.I can only speak for end game mythics, and based on what I read on this forum:
- 5 hunters or warlock are fun and, as of today, allow you to finish 10+ keys for some dungeons.
- Same for 5 prot paly but easier although less fun. Also key levels are a bit lower.
- resto Druid, guardian Druid and 3 bm hunters to finish the highest keys a multiboxer could do. Difficult compo for a beginner.

It is possible that the compos above become irrelevant for bfa as the classes might change too much. It’s my understanding that 5 tanks became a thing after people started playing the beta for legion. With bfa in alpha, it’s unlikely that someone could advice a bfa proof compo.

Good luck!