Both offense and stall in OU got some interesting new toys to play with.
On the offensive side of things, Mega Metagross immediately comes to mind. What was once an underwhelming base 70 speed has been patched up to a great base 110. With a great base 145 attack and Tough Claws to boost the power of Bullet Punch, Meteor Mash, Zen Headbutt, elemental punches, etc., it has no problem putting on the hurt, and 80/150/110 defenses make it pretty bulky as well. So now we've got something that's bulky, strong, and fast. It can rock an all-out attacking set without much duress, run a classic AgiliGross set to outspeed every choice scarf user, or whatever. Definitely an all-around good Mega, although it wishes it still had neutrality to Dark and Ghost. Oh well.
Then there's Mega Gallade, who may very well cause Mega Medicham to fall out of the tier again. While it's not as immediately powerful as Mega Medicham, and of course we all really hoped it had something better than Inner Focus, it boasts better bulk, and a better speed tier with a solid base 110. Plus, despite Mega Medicham having higher attack, base 165 attack is still stupidly good, and with access to SD, it can boost its attack to stupid levels. It's got a solid offensive movepool to abuse all that power as well, with plenty of surprise coverage moves depending on what you wanna chew up. It pretty much entirely outclasses Mega Medicham, who has very little to set itself apart, barring the ability to 2HKO Unaware Clefable.
On the defensive side of things, despite being so heavily criticized about its design and its ability, Mega Slowbro stands supreme. It's true that losing Regenerator for Shell Armor is a rather lame trade-off, but it's still got reliable recovery in the form of Slack Off. Plus, having an insane base 180 defense speaks for itself. It can either run a physically defensive set to make it pretty much impossible to 1HKO with a physical attack, or a specially defensive spread to be a solid mixed wall. Plus, unlike a lot of defensive Pokemon, sporting a base 130 special attack is rather impressive, and we're all familiar with Slowbro's attacking options as well, with Scald for defensive burns and coverage moves including Ice Beam, Psyshock, and even Fire Blast. True, Toxic can be a nuisance to it, but that's not enough to undermine its fantastic defensive potential.
And you know who Toxic isn't a nuisance to? Mega Sableye. This thing is at home both on Stall teams, but also against Stall teams as well. In fact, a lot of things on Stall teams have trouble dealing with this thing. Although a base 50 HP lets it down a bit, base 125 defense and base 115 special defense are both solid, it's got reliable recovery with Recover, and with Magic Bounce to reflect things like Toxic, Will-o-Wisp, Hazards, and Roar/Whirlwind, it can be a major nuisance to stall. Plus, between Calm Mind and Will-o-Wisp, it can make itself even more annoying to kill as well. Still, low HP compromises its bulk, its speed is nonexistent, and it's not immediately threatening without setting up.