For a playthrough, Zygarde has Eternatus beat. It has a higher BST when in Complete Form, and is more offensively-based. It gets excellent moves like Thousand Arrows, Outrage, and Crunch, meaning it can do a lot of damage to any foe. Eternatus works, but Zygarde is better.
Zygarde @ Life Orb / really anything else
Ability: Power Construct
- Thousand Arrows
- Coil / Brick Break
Eternatus is quite good offensively as well, it's true, but I do prefer Zygarde. I may as well provide a moveset for Eternatus as well, in case you would prefer a special attacker.
Eternatus @ Life Orb / Choice Specs / Wise Glasses
- Dynamax Cannon
- Sludge Wave
- Flamethrower / Mystical Fire
- Flash Cannon / Shadow Ball
Hope I helped!