After playing on Showdown for a while, I've seen several Pokemon that are definite threats this generation.
-Mega Mawile: Like stated multiple times in this thread, this Pokemon is an absolute monster. I haven't gone up against an opposing Mega Mawile, but I use it on one of my teams and it's been able to destroy teams. While Fire types may help against it, Sucker Punch hits very hard when you factor in Huge Power and potentially Swords Dance.
-Aegislash: This is a pretty nerve wracking Pokemon to go up against. The combination of its ability, King's Shield, Swords Dance, and STAB priority in Shadow Sneak allow it to be a top tier threat. If you let it set up it can devastate you, though a couple Pokemon are able to take it out relatively easily. Volcarona, Garchomp, Salamence (Physical and Mixed sets work due to Earthquake and Fire Blast), and Mamoswine come to mind. I've also seen someone use a Focus Sash Gengar against it - Have the Aegislash Shadow Sneak and go into offensive stance and take it out with Shadow Ball.
-Mega Lucario: This is very good, certainly a huge offensive threat as of now. Adaptability Close Combat coming off that Attack stat is huge, as is Bullet Punch. It does need to be wary of anything faster, especially if it's using Extremespeed instead of Bullet Punch.
-Talonflame: I didn't really expect Talonflame to see much usage in OU but with the addition of Gale Wings, it's terrifying to face. Priority Brave Bird, especially after a Swords Dance, gives it huge offensive presence and a lot of pressure on your opponent. Acrobatics also works, though I've seen Brave Bird much more. One thing to watch out for with Talonflame is obviously Stealth Rocks, so be sure to pack a spinner.
-Trevenant: Speaking of spinners, this is the worst nightmare of one. Trevenant may not seem like such a threat, but when you look closer, it's actually great as a spinblocker. Its typing and bulkiness allows it to stare down spinners and take them out. It can come in against a Starmie's Ice Beam and take another while OHKOing it with Wood Hammer (Though the recoil would kill Trevenant). With Natural Cure, it has access to a great recovery move in the form of Rest, though it obviously has to switch out. If you don't want that, you could grab one with Harvest and equip it with a Sitrus/Lum Berry, which is a great form of recovery. It's obvious that some Pokemon could come in on it and set up, namely special attackers due to Will-O-Wisp, but Trevenant's a great spinblocker that I think will be a defensive threat this generation.
Noivern: While not particularly strong, Noivern is very fast. Base ~123 is nothing to scoff at, and with the help of Life Orb or Choice Specs it can hit fairly hard. This is also due to Boomburst - A base 140 100% accuracy move is absolutely terrifying. Also, Noivern's abilities are fairly useful - Frisk allows you to scout out an opponent's item to better judge its set while Infiltrator lets you bypass Screen moves and Substitute. Though Noivern has many tools to be successful, it's not without its weaknesses - It doesn't particularly enjoy losing 25% of its health upon switching in and priority moves like Ice Shard and Bullet Punch absolutely wreck it. Bulky Pokemon also do fairly well against it due to Noivern's relatively subpar Special Attack. I still think that it will be a threat, though.
