Zygarde's pros and cons: Yea, echoing gamer boy, Zygarde is really cool but flawed. Not a stellar attack (base 100 isn't bad but it isn't great), decent-but-not-great speed (base 95), average special attack (base 81), are all not great signs, and Zygarde's ability is completely useless in OU (and Ubers). However, not everything is bad for this Dragon/Ground legend: it's got FAT bulk (bulkier than Mega Garchomp, which is very bulky, and Hippo), no real awful stat, the much-loved EdgeQuake combo, and strong (albeit nonSTAB) priority in the form of Extreme Speed. The real killer is how shallow Zygarde's movepool is: no fire coverage other than HP is a killer, especially since you are working with basically the same special attack as normal Tankchomp, so losing the ability to nail Ferro or Skarm (especially the latter, which resists Espeed/Edgequake/Dragon Tail/Claw/Outrage) is annoying on sets other than SubCoil (and even then giving Ferro free rocks/spikes is never fun).
On Zygarde's Movepool: Pretty much the only viable move Zygarde has that isn't used often is
Glare. Paralyzing any non-electric type including popular/good mons like Landorus-T, Hippowdown (although you really want to Toxic this guy), Garchomp, Gliscor, is supremely useful given that several of the prior mons can be dangerous sweepers. But, Zygarde can't really utilize Glare on anything other than a bulky ParaShuffler set simply because there's just not enough slots.
On Coil sets and Iron Tail versus Stone Edge: Coil sets are awesome, since they let you set up on things like Mega Venusaur, Rotom-Wash (with some luck), Hippo/Skarm, Ferro, and so on, and with minimal speed investment you can outspeed and outphaze them with Dragon Tail. Good late-game win con, but still susceptible to strong physical attackers before you set up, and strong HP Ice's (Manectric, Raikou, etc.). Also, is hard walled by most fairies since you run EQ/Dtail on defensive Coil sets, and offensive Coil sets must choose between Iron Tail (which can work against fairies) or Stone Edge (which gives you much more coverage on average).
The debate between Iron Tail and Stone Edge is somewhat personal preference, but I feel that for
most teams, Stone Edge>Iron Tail because while Iron Tail does hit fairies, you really don't want to stay in against any fairy other than an un-setup Clefable, and you'll still take over 50% in damage. Iron Tail is also inaccurate, and you lose coverage on important things like Charizard Y, Volcarona, Talonflame, and most other flying types which you normally check or counter really well.
On DD sets, and good EV benchmarks for them: As for DD sets, I think going for a mix of bulk and attack/speed is necessary, and it depends on the team in question. For speedier teams, you generally can get away with an Adamant Nature Zygarde with 240 Speed EVs, since that outspeeds your standard Mild Nature Kyurem with 236 Speed EVs by 1 point, and your standard Scarf Kyurem-B by a bit when Zygarde is at +1. In addition, Adamant with 240 Speed lets you outspeed Mega Manectric and Mega Lopunny after a DD, which is nice since even with modest Attack investment you can 2HKO with EQ or kill with EQ+Extremespeed.
Alternatively, Jolly Zygarde with 164 Speed EVs outruns Mega Alakazam and Mega Aerodactyl at +1, but the latter isn't very common and the former can't OHKO you even with HP Ice, letting you win the 1v1 with EQ+Espeed.
IMO, one of the key things to do when EVing Zygarde is try and make use of that amazing bulk. 108/121/95 is one of the bulkiest mons in OU, and Dragon/Ground is a solid defensive typing (while not amazing and having key weaknesses in Ice/Dragon/Fairy).
Here's the DD Zygarde set I use.
Zygarde @ Life Orb
Ability: Aura Break
EVs: 120 HP / 148 Atk / 240 Spe
Adamant Nature
- Dragon Dance
- Stone Edge
- Extreme Speed
- Earthquake
As for the EV spread, it's a bit different than the normal one. 240 Speed for the reasons mentioned earlier, and 120 HP gives you the bulk you need to set up on a lot of things and check/counter threats that an uninvested Zygarde simply can't. Even though your attack has a piddly 148 (300 attack with the EV investment) EVs invested, with the Life Orb and a DD, you start hitting hard enough, especially since you have +2 priority in the form of Extreme Speed.
With this spread, you can do things like set up on Choice Scarf Keldeo locked into Secret Sword if needed (a little over 60% of the time), 1v1 Charizard Y in the Sun even with rocks on your side, eat an Adamant Mega Charizard X Dragon Claw from full and live 100% of the time and OHKO back with EQ (this has a 50% shot to kill you due to recoil, but that's still impressive), eat an Adamant Mega Medicham High Jump Kick from full and live 100% of the time (EQ does minimum 76% back, and Extreme Speed 41% minimum, so you can revenge kill with just a bit of chip damage and be KO'd with recoil, or have something wear down Mega Medicham and then use Espeed to finish it off).
If you want to avoid the nasty recoil and stay in longer, swap out Life Orb for Leftovers. However, the power drop is very noticeable, and this Zygarde set is meant to sweep once an enemy team is a bit weakened and has things like Rotom-W out of the way. Alternatively, you can run Lum Berry and use that to set up on things like Mega Sableye and Suicune (Scald is a 5HKO and at +2 you cleanly 2HKO Suicune).
The set's bulk lets it set up on common threats/checks like AV Tornadus, Offensive and Defensive Mega Venusaur (you'll ideally want Offensive to be weakened though), ScarfTar/MegaTar/SupportTar/BandTar not locked into Crunch, and various other mons. Unlike Dragonite, Zygarde resists SR, which lets it have a few more setup chances against certain mons (Tyranitar is one of the biggest), and you're bulkier by a noticeable amount.
