252 Attack, 252 Speed, 6 HP
Outrage/Dragon Tail/Stone Edge
Most People think of Zygarde as Inferior to other OU Dragon Types. This isn't really
true; it's just that Zygarde does things differently and uniquely, and thus is outclassed by none.
If you've actually used Zygarde you will know that Coil is easy to set up thanks to Zygarde's great HP and Defense Stats, allowing it to set up against a host of physical threats. You generally will not usually be using Zygarde's weakness on the physical side unless you get STAB on them (Gliscor and Donphan say hi but Zygarde can actually still set up on them regardless) While Zygarde has priority in Extreme speed, often, Substitute tends to be a better choice. Zygarde doesn't hit very hard with E-Speed until it sets up, while Substitute protects it against status such as Will-o-wisp and can turn Pokemon such as Ferrothorn into setup fodder. After a few coils, Zygarde is very much capable of reaching a point where it's Substitutes will not break. Additionally, it can help against being revenge killed, which is the main reason as to why you might want to use E-Speed in the first place. Earthquake is Self-Explanatory, unless you want to use Land's Wrath for novelty STAB. Zygarde does not learn Dragon Claw, so Outrage is it's only powerful secondary STAB. Alternatively, you can use Dragon Tail, which will still hurt and can prevent you from being phazed. It will also let you beat out apposing boosters. Stone Edge forms the EdgeQuake combo, and has perfect accuracy after a Coil, so it's a safe bet to go on if you dislike Outrage and Dragon Tail's negative effects.
Zygarde is generally a physically bulky booster, which separates it from Garchomp. The only other "Bulky" Booster is really just Dragonite, who has a Stealth Rock Weakness, is only bulky when at full HP, and Lacks STAB Earthquake (Fun Fact: Zygarde's Earthquake will out-damage Dragonite's Dragon Claw if both run the same nature)