Single Battle - Earthquake. You don't need to worry about allies because you have none!
Double Battle - Land's Wrath, unless you have a Flying Type, Levitate User or a Pokemon with Telepathy.
Triple Battle - Land's Wrath, unless you have two Flying Types, Levitate Users or Pokemon with Telepathy. Then choose Earthquake.
Land's Wrath only has 10 less power than Earthquake, and Zygarde is one of the Pokemon that is taking over the doubles scene with it.
Source: Experience in Double Battles