Mega Silvally Dark
Ability: Adaptability
Stats: 95/125/95/125/95/135
New Moves: Knock Off, Sucker Punch, Recover
Justification: Dark types have a lot of different utility STAB moves that they tend to run in the same set, so Adaptability supports this. Knock Off, Pursuit, Parting Shot, and Defog all give it a lot of utility that's pretty good on such a fast, bulky mon. Swords Dance also looks pretty scary.
Mega Silvally Ghost
Ability: Levitate
Stats: 95/105/95/170/95/110
New Moves: Will-o-Wisp, Recover, Hex
Justification: Basically just uses Silvally's bulk and movepool to function as a powerful offensive pivot. Levitate helps it deal with Landorus-T, and avoid Spikes, making it a pretty good offensive Defog user.
Mega Silvally Dragon
Ability: Intimidate
Stats: 95/145/100/125/100/105
New Moves: Dragon Dance, Earthquake, Recover
Justification: Kind of like old school Salamence, it has the potential to go for a Dragon Dance sweep or function as a mixed attacker. Intimidate gives it the bulk to either set up or function as an effective pivot.
Mega Silvally Fairy
Ability: Serene Grace
Stats: 95/105/105/115/155/95
New Moves: Heal Bell, Moonblast, Recover
Justification: Very tanky version that uses Serene Grace Moonblast to make it almost inpenetrable on the special side.