Adamant Silvally-Poison with 252HP/252Atk/4Spe - those 4 Spe EVs allow Silvally to outspeed 0 Spe Tapu Lele, guaranteed OHKO after rocks, 75% chance to kill without. Outspeed and OHKO Tapu Bulu, Slower than but survives Tapu Koko and OHKO in return, faster and Guaranteed 2HKO on Tapu Fini. Walls out Pharimosa and OHKO. Thats a lot of common threats neutered by Silvally.
There are a lot of flaws within those statements. Firstly, 4 Spe EVs do not outspeed any form of tapu lele whatsoever. Tapu Lele will always run timid max Spe since that is its best set. As a result of that it will outspeed and OHKO your silvally with psychic unless it switches in on it and lets be honest, nobody switches in a tapu on a poison type. Next we have the tapu bulu statement, which seems to be true for the most part. Thirdly, tapu koko does not fear silvally at all, it can just volt switch/u-turn on it and pick up the KO the next time, because silvally lacks the much needed recovery. Next, 4 Spe EVs does in general outspeed tapu fini as most fini sets hit the 220 Spe benchmark (which outspeeds jolly Azumarill), but keep in mind that max speed taunt cm is a popular set although it probably cant touch silvally. lastly, yes silvally does take Phero on, but phero just u-turns out of there, which leaves you vulnerable to get trapped by dugtrio (which is a very popular pairing, as dugtrio deals with toxapex). From that I would conclude that poison silvally is not the most reliable fairy check in pokebank OU.
As for my own experience with silvally, i can conclude that most normal type sets outperform the memory sets, albeit that the normal sets are often outclassed by other mons themselves. So I went looking for something that only Silvally can uniquely do:
Silvally @ ground memory
careful nature
EVs: 248 HP, 200 SpDef, 60 Spe
multi-attack
ice beam
parting shot
thunder wave
The purpose for this set is to fill in a support role by spreading the t-wave as much as possible (silvally and zygarde are the only ground types that can spread paralysis apart from groudon) and gaining momentum. Ground/ice grants you awesome coverage, you naturally force out the electric types and ground types sustain significant damage from the ice beam (especially lando-t, garchomp and zygarde). Parting shot is there because its the best move silvally gets and allows it to run from bad matchups(cool trick: parting shot against Msableye will force it out).
Running SpDef allows you to counter all variants of tapu koko (and fast electrics in general) except for the grass knot (100bp) ones. It also allows you to live a hit from tapu lele so you can t-wave it in return. Careful nature is chosen as the alternative is gentle nature(+SpDef, -Def) and in practice you will be taking physical hits, albeit that Spa neutral nature allows you to OHKO offensive lando-t after rocks, but all other cases remain 2hko's regardles of nature. 60 Spe allows you to outrun neutral base 70s (bisharp, breloom) and positive base 60s (incineroar, primarina, magnezone(?)). I think outspeeding bisharp is very important as it can abuse your parting shot, but if facing a +3 bisharp is not a problem to your team, then feel free to go full SpDef. Running no Speed still allows you to outrun stuff that hits the 220 benchmark like rotom-w(which completely shuts you down), tapu fini and defensive lando-t.
I am not claiming that this set is the be all end all set for Silvally, but it is at least a niche that only Silvally can do, thus making it automatically the best at it. Maybe that isn't even true as sub/coil/glare/thousand arrows zygarde might be able to do the same thing (spreading paralysis and beating ground/electric types), but with acces to leftovers. At the very least, Silvally is a pivot and not 4x weak to ice, which Zygarde isnt.
