Mix-and-Mega Viability Rankings!!!!1
That's right, we're doing this again.
S Rank: Reserved  for Pokemon that are at the top of mix and mega's metagame. These  Pokemon are able to perform a variety of roles very effectively, or can  just do one extremely well. Their use has low risk involved and high  reward exerted. Pokemon in this rank have very few flaws that are  patched up by numerous positive traits. These Pokemon are here as top  threats and potential Bans due to using multiple megastones easily or  just one very well.
S Rank
 Mew (Ampharosite, Lopunnite, Pidgeotite, Diancite)
  Terrakion (Lucarionite, Diancite, Pinsirite, Lopunnite)
 Primal Groudon (Red orb)
A Rank: Reserved  for Pokemon that are fantastic in the Mix-and-Mega metagame, and can  sweep, support, or wall significant portions of the metagame. These  Pokemon require less support than most others to be used effectively and  have few flaws that can easily be overlooked when compared to their  positive traits.
A+ Rank
 Victini (Red Orb, Cameruptite, Pidgeotite)
 Blissey (Sablenite, Slowbronite)
 Thundurus (Manectite, Pideotite, Altarianite)
 Archeops (Aerodactylite, Charizardite X, Pinsirite, Salamencite, Lucarionite)
 Gengar (Absolite, Manectite, Diancite, Pidgeotite)
 Landorus-T (Lopunnite, Mewtwonite X, Altarianite, Salamencite)
 Manaphy (Sceptilite, Absolite)
 Zygarde (Altarianite, Pinsirite)
 Arceus-Normal 
A Rank:
 Heatran (Latiasite, Absolite, Pidgeotite, Red Orb, Altarianite, Cameruptite)
 Entei (Pinsirite, Altarianite, Aerodactylite, Red Orb)
 Arcanine (Salamencite, Altarianite, Banettite, Pinsirite, Red orb)
 Gyarados (Salamencite, Pinsirite, Charizardite X, Blue Orb, Aggronite)
 Weavile (Glalitite, Aerodactylite)
 Metagross (Pinsirite, Aerodactylite, Diancite)
 Kyogre 
A- Rank:
 Mamoswine (Lucarionite, Glalitite)
 Keldeo (Lucarionite, Blue Orb, Pidgeotite, Absolite, Manectite)
 Diggersby (Medichamite)
 Ferrothorn (Blue Orb)
 Kyurem (Glalitite, Diancite, Aggronite, Absolite, Sablenite, Camerupite, Pinsirite)
  Latios(Altarianite, Sceptilite, Manectite, Absolite, Lucarionite, SoulDew)
 Noivern (Gardevoirite, Pidgeotite, Salamencite)
 Suicune (Aggronite, Slowbronite, Sceptilite, Blue Orb)
 Hippowdon (Red Orb, Sablenite, Slowbronite)
 Excadrill (Lucarionite, Pinsirite)
 Zapdos (Manectite, Pidgeotite, Sablenite, Venusaurite)
 Skarmory (Blue Orb, Venusaurite)
 Xerneas 
B Rank:  Reserved for Pokemon that are great in the Mix-and-Mega metagame. These  Pokemon have more notable flaws than those above them that affect how  they function in the tier. Their positive traits still outshine their  negatives, but they require a bit more team support to bring out their  full potential.
B+ Rank
 Blaziken (Blazikenite)
 Tornadus (Pidgeotite)
 Tyranitar (Sharpedonite, Steelixite, Garchompite, Diancite, Pinsirite, Salamencite)
 Staraptor (Lopunnite, Pinsirite, Mewtwonite X, Aerodactylite)
 Snorlax (Banettite, Metagrossite, Altarianite, Salamencite)
 Cobalion (Aerodactylite, Lucarionite, Galladite)
 Garchomp (Gyaradosite, Diancite, Banettite, Scizorite)
 Giratina (A)
 Togekiss (Gardevoirite, Aggronite)
 Aegislash
 Slowking (Sceptilite, Slowbronite)
 Slowbro (Slowbronite, Aggronite, Sceptilite)
 Mewtwo (Mewtwonite X, Mewtwonite Y)
 Gourgeist-Super (Aggronite)
Hoopa-Unbound (Manectite, Diancite, Absolite, Aggronite)
 Jirachi (Metagrossite, Sceptilite, Ampharosite, Diancite, Manectite, Absolite)
 Ho-Oh
B Rank
 Kangaskhan (Kangaskhanite)
 Mienshao (Lopunny, Altarianite, Pinsirite, Aerodactylite)
 Porygon-Z (Diancite, Altarianite, Pidgeotite, Absolite, Manectite)
 Latias (Soul Dew)
 Azumarill (Mawilite, Medichamite)
 Rypherior (Heracronite)
 Infernape (Banettite, Diancite)
 Azelf (Red Orb)
 Scizor (Blue Orb)
 Breloom (Banettite)
 Lugia
 Volcarona (Red Orb, Sablenite)
 Vaporeon (Aggronite, Sceptilite, Sablenite)
 Meloetta (Slowbronite, Lopunnite, Manectite)
 Yveltal 
 Genesect
 Deoxys Attack
 Arceus Ghost 
B- Rank
 Roserade (Red Orb, Scizorite, Pidgeotite)
 Flygon (Pinsirite, Altarianite)
 Giratina (O) 
 Salamence (Salamencite, Pinsirite)
 Alakazam (Pidgeotite, Absolite, Manectite)
 Bisharp (Metagrossite, Lucarionite)
 Haxorus (Gyaradosite, Metagrossite, Aerodactylite)
 Shaymin-Sky
 Darkrai
 Magnezone (Latiasite, Cameruptite, Diancite, Manectite)
 Starmie (Pidgeotite, Blue Orb, Sceptilite, Altarianite)
 Rayquaza 
 Krookodile (Blue Orb, Altarianite, Diancite)
 Milotic (Sceptilite)
 Heliolisk (Sceptilite, Manectite, Blue Orb)
 Kyurem-Black
 Kyurem-White
 Deoxys Speed 
C Rank: Reserved for  Pokemon that are mediocre in the Mix-and-Mega metagame, but have just as  many notable flaws that prevent them from being effective. Pokemon in  the C tier often require significant support to be effective. Pokemon  from this rank tend to face a lot of competition with the more commonly  used Pokemon. These Pokemon exert a below average presence in the current metagame.
C Rank
 Hydreigon (Blastoisinite, Manectite, Gardevoirite)
 Gorebyss (Banettite)
 Conkeldurr (Gyaradosite, Pinsirite, Aerodactylite, Charizardite X)
 Chandelure (Red Orb, Cameruptite)
 Goodra (Ampharosite, Gardevoirite)
 Dialga 
  Palkia 
 Dragonite 
 Rotom-Wash (Manectite)
 Reshiram 
 Zekrom
 Darmanitan (Diancite, Pinsirite, Altarianite)
  Deoxys Defense 
D Rank: Reserved for Pokemon that are mediocre in the Mix-and-Mega metagame, but are viable enough to justify their use on select teams. These Pokemon are either usable but have no real niche, or are only capable of doing their specific task and fail at doing anything more than that. Pokemon from this rank have multiple crippling flaws that prevent them from being successful a majority of the time, and are often severely outclassed because of it.
D Rank
 Shaymin (Lucarionite, Manectite)
 Reuniclus (Cameruptite)
 Drifblim (Banettite)
 Diancie (Altarianite, Diancite)
 Cresselia 
 Greninja 
 Meganium (Sablenite, Audinite)
 Deoxys Normal 
 Arceus Steel 
''CONCLUSION REACHED'' POKEMON: Pokemon in this list have reached a conclusive ideal ranking, so unless the metagame changes towards them or there is something about them that hasn't been said, discussion about them is disallowed.
BLACKLISTED POKEMON: Pokemon that are not only unviable in Mix-and-Mega, but also make the thread shitty whenever they are brought up because most people that argue about them getting ranked are inexperienced players using bad arguments.
Rules: 
- Post Intelligently. Your argument should consist of the pokemon's effectiveness relative to the tier's threats, and how it fits into a certain ranking.
- NO flaming over rankings. OM has had an issue before with some arguments getting a little heated, but I will ask a moderator to delete any post containing an argument that is remotely insulting.
- Usage/how common something is should not be used in your argument. Viability rankings isn't what mons are the most used, its what's the most effective.
- Eevee General is beautiful. This is an undisputed fact.
Notes
- Thanks to Dr. Phd. BJ, AllJokesAside, Ransei, SpartanMalice, InfernapeTropius11 for helping me do the rankings!
 
- All ranks are up for debate atm. Nominate a mon if it is not listed, or nominate movements. 
 
- Any and all nominations can be rejected for any reason if there is consensus it is a bad nomination. 
 
- If you nominate a new mon, be sure to include ALL stones it can viably use, unless some stones are very niche in comparison to their ranking.
- Updates may be slow, please be patient; I will update this in bulk.