Currently, Zygarde-Complete is the second-highest rank Dragon-type in the SS Ubers Viability Rankings. In the usage stats, it's #10 and has maintained that average for a long time. It's one of the best Pokémon in the SS Ubers metagame in its own right, being an offensive and defensive behemoth in the tier. It's one of the three ranked Dragon-types in the S and A ranks, and it anyways defies its competition in the role that it can play.
Zygarde's one of the most terrifying Pokémon in the tier, and one of the most lethal, and with good reason. A lot of things have gone its way recently, especially the fall of Tangrowth and Buzzwole and Zacian's ban. The popularity of so-called checks, such as Ferrothorn, doesn't stop Zygarde from running haywire as it can easily fold them with its versatile offensive sets. Its offensive sets are notorious for eliminating its best answers. Zygarde's ability to set up a Substitute against most of the metagame is allowing it to easily set-up on a lot of defensive walls, which would be forced to answer it in a way that they wouldn't like. Ho-Oh is invalidated in its phazing attempts while switching in is proven worthless a lot of times as it just gets phazed right back out into more setup fodder for Zygarde. This loop is augmented even further by the increasing presence of Ferrothorn as a teammate for Zygarde, as it provides the Spikes support and added offensive pressure. It also haxes people a lot, which makes it look scarier than it is.
Defensively, Zygarde is a great tool to compress in offensive teams and on balances. Zygarde's Dragon-/Ground-type is a great asset as it typically permits it to run a defensive set which can easily check the likes of Marshadow, Groudon, physically offensive Yveltal and most of the other physical Pokémon in the metagame. Groudon's rise brings further light to the physically defensive set, as it's one of the only things that can handle almost all of Groudon's sets. Its signature ability, Power Construct, gives it a lot of bulk and free HP after transforming. It’s one of the Pokémon that makes a great switch-in to Necrozm-Dusk Mane. Glare can paralyze even Ground-types, which is useful. By extension of people not prepping for Groudon, a lot of players do not prep for Zygarde as well (at least not enough), which Zygarde benefits a lot from. Zygarde also benefits from many players who ignore putting a cleric on their team, which is a big factor for balance teams.
Zygarde is one of the most versatile Pokémon in the tier, especially when seeing how its checks usually change. Physically defensive Xerneas is a solid set, but it's hard to justify a lot of times, and it's not common enough. Tangrowth and Buzzwole fell hard recently, and they don't provide any other utility than being a soft check to offensive Zygarde which can muscle past them late-game. Other Dragon-types can't switch-in once it's boosted, and most phazers get worn down easily. Other checks are gimmicks like Shedinja. It also has a few of the best moves in the metagame. A combination of Glare and Thousand Arrows is enough to pile pressure on the opponent. Set-up options in Dragon Dance and Coil increase its offensive prowess by making it difficult to predict its set. It also has the Haze + Glare support combination, which is solid if you can get it to Complete form and once it's in, eliminating it becomes way harder.
