I thought I'd join this conversation, about Xerneas - a Pokemon that I think belongs in The S-Tier in VGC 16.
So obviously the S-Tier is defined as
"
Reserved for the top threats in the VGC16 metagame. The Pokemon in this tier are able to perform multiple roles to tremendous effect, whilst having few to no flaws. Support Pokemon in this tier can easily create free turns for their teammates without creating clear openings for the opponent. Any flaws that these Pokemon have can be taken care of with little to no team support."
Initially it is easy to prove Xerneas as a top threat in the metagame.
Via Braverius' blog, The usage of the top 25 teams in the format (taken once a week - at least for the 2 weeks of the format was undertaken)
http://braverius.blogspot.com.au/se...-max=2016-01-01T00:00:00-08:00&max-results=14
Across the teams that are currently having the most success, Every single team either utilises Xerneas, or values its spot in the metagame so high that it has at least two resistances to fairy type moves.
Through using this data, it can be observed that the most successful teams in this format recognise both Xerneas' value to their own teams as well their opponents, and the level of usage / countering would be indicative to a Pokemon belonging in the S tier.
Further, another quality which would leave xerneas in the S tier, would be to act as a supporter - in creating free turnsfor its teammates.
Xerneas is able to achieve this through the threat of its unique move. Geomancy. Geomancy is a move offering great pressure of a early, mid or late game sweep, through xerneas' access to powerful spread ( or single target) fairy moves (Dazzling gleam and moon blast) boosted by its unique ability fairy aura - essentially a free life orb.
The very threat of Xerneas on the field, is enough to see double targets, taunts, fake outs, knock offs, land into the slot in an effort to stop a likely early game loss. However all of this is minimal compared to the opponent's action when xerneas has indeed set up. Teammates are essentially free to do as they wish unless the opponent is willing to make hard reads, calling defensive Xerneas play - which may result in a game loss.
The final qualifying factor, regarding flaws and how they may be resolved is a matter shown with simplicity.
Xerneas has a Basestat of 680, featuring Defensive stats of 126 HP, 95 DEF. 98 SPDEF
As Xerneas is an "uber Pokemon" not much is able to ko it before it is able to set up and sweep. Notable exceptions include Aegislash and Ferrothorn, which still require boosting items and/or xerneas to gain speed before a ko can be reached.
252+ Atk Aegislash-Blade Iron Head vs. 28 HP / 12 Def Xerneas: 170-204 (82.9 - 99.5%) -- guaranteed 2HKO
252+ Atk Ferrothorn Gyro Ball (104 BP) vs. 28 HP / 12 Def Xerneas: 162-192 (79 - 93.6%) -- guaranteed 2HKO
It can be noted that there is an absence of Pokemon that both outspeed Xerneas and are able to get large damage off on it, due to its large bulk, this makes using fake out (to neutralise one Pokemon on a given turn) and redirectors easy features for any VGC team, but especially Xerneas teams which remove its weaknesses to strong super effective single target attacks. It is also seperated from contemporary set up sweepers as it gains the ability to "stay around" with the boosts it ascertains. A geomancy gives Xerneas the equivalent defensive boost of an Amnesia, making it even more troublesome to truly counter, as the weakness it has to poison and steel moves, generally comes in the way of special attacks. Therefore, it is essentially able to defeat the Pokemon that may have countered it, with nothing but a free turn.
I think it's fair to conclude, Xerneas whilst on paper seeming like a simple set up and sweep Pokemon offers much more pressure then one would expect from a simple A-Tier Pokemon. On team preview, one cannot under prepare for a Xerneas lead. Failing to lead correctly into Xerneas + Support = Game loss. Through the popularity of Xerneas (And the efforts made by successful teams in stopping its run), the large presence it has on the field helping it support its teammates as well as the lack of threats that can deal with the barest support, Xerneas should retain a place in the S tier - at least at this stage in the metagame.