It all depends on the scenario. Elemental shaman is very if this this and this then do this, but if one thing changes everything changes.
3+ target spam earthquake, try to keep three flame shocks up and use lava surge, with chain lighting as filler of course. If the targets will be up a while.
If they are low health or won't be up long I would use Earth Shock on the highest priority, usually the healer or caster of the group (can change depending on affix in mythic +)
I would go to http://stormearthandlava.com/ (elemental shaman dedicated website) very helpful for your abilities, even if you aren't using the meta build