I can't speak so much for Ubers, but in Gen 6 BSS Mega Mawile was flat-out scary, and I see no reason that would change in Gen 7 (once Mawile is available, that is.) If you played BSS at all, this set probably seems familiar:
Ability: Intimidate (-->Huge Power)
EVs/Nature: 252 HP / 252 Atk / 4 Spe, Adamant
- Swords Dance
- Play Rough
- Knock Off / Iron Head / Fire Fang
- Sucker Punch
Pretty simple stuff here. Mega Mawile has such incredible Attack after Huge Power that this set can mow down even defensively oriented teams. If something with really high Defense that resists your attacks gets in your way, use Swords Dance. Mawile is immune to Toxic, so it can handle some stall attempts quite well. Play Rough is obvious and poorly-resisted STAB, Sucker Punch makes up for Mawile's subpar speed and has solid coverage with Play Rough. Knock Off has incredible utility and doesn't need your opponent to use a damaging attack, but Iron Head is another STAB and Fire Fang rounds out coverage with Play Rough nicely.
Mega Mawile is pretty simple, but its low Speed and difficulty with some Steel- and Fire-types mean that it's far from invincible. Be sure to keep up those Stealth Rocks, because otherwise you'll be bait for the Mega Charizards.
The risk I took was calculated, but man, am I bad at math.