So far I'm probably most torn between Spiritomb and Glalie.
Yveltal in Ubers has already shown that many very useful moves for either a defensive or offensive mon get that Dark Aura Boost, and Spirtomb's bulk is already at least as good as Rotom-W. STAB Dark Aura Foul Play does a number on a lot of OU, since many of its neutral targets, investing or not, have decently high attack stats naturally, not to mention the abundance of physical attackers already.
0 Atk Dark Aura Spiritomb Foul Play vs. 0 HP / 4 Def Thundurus: 186-219 (62.2 - 73.2%) -- guaranteed 2HKO
0 Atk Dark Aura Spiritomb Foul Play vs. 0 HP / 4 Def Landorus: 162-192 (50.7 - 60.1%) -- guaranteed 2HKO
0 Atk Dark Aura Spiritomb Foul Play vs. 0 HP / 4 Def Mega Metagross: 318-374 (105.6 - 124.2%) -- guaranteed OHKO
0 Atk Dark Aura Spiritomb Foul Play vs. 56 HP / 0 Def Landorus-T: 264-312 (79.2 - 93.6%) -- guaranteed 2HKO after Leftovers recovery
0 Atk Dark Aura Spiritomb Foul Play vs. 120 HP / 0 Def Talonflame: 201-237 (61.4 - 72.4%) -- guaranteed 2HKO
0 Atk Dark Aura Spiritomb Foul Play vs. 252 HP / 104+ Def Mew: 224-266 (55.4 - 65.8%) -- guaranteed 2HKO after Leftovers recovery
0 Atk Dark Aura Spiritomb Foul Play vs. 252 HP / 0 Def Jirachi: 272-324 (67.3 - 80.1%) -- guaranteed 2HKO after Leftovers recovery
There's at least a few good targets for those. Not sure he'll find too much to use Sucker Punch or Pursuit against, but they're options.
My one issue with Glalie is that, while potent offensively, Ice is still a horrendous defensive typing, and his overall coverage is mediocre. Unlike, say, Zard-X or Altaria, Glalie isn't going to find itself able to possibly tank a hit to get up more than one boost, especially since its bulk isn't any better either. Refrigerate boosted moves do give it decent coverage with STABs and EQ, but while it makes it easier for Glalie to sweep, many of the things that walled him before still Wall him now.
Dragon Dance Mega Glalie: This is another Pokemon where I wish the council would have picked a better buff for. Both flavor wise and competitively, Shell Smash would have been a better choice. Glalie can still be checked by most of the same stuff as before even with Dragon Dance.
0 SpA Glalie Freeze-Dry vs. 248 HP / 0 SpD Rotom-W: 120-144 (39.6 - 47.5%) -- 2% chance to 2HKO after Stealth Rock and Leftovers recovery
0 SpA Glalie Freeze-Dry vs. 0 HP / 4 SpD Keldeo: 138-164 (42.7 - 50.7%) -- 42.2% chance to 2HKO after Stealth Rock
+1 252+ Atk Glalie Earthquake vs. 0 HP / 0- Def Mega Metagross: 204-240 (67.7 - 79.7%) -- guaranteed 2HKO after Stealth Rock
At least with Shell Smash, Mega Glalie would have the possibility to sweep once all priority users are gone. Sigh...
Should look more like this
0 SpA Mega Glalie Freeze-Dry vs. 248 HP / 0 SpD Rotom-W: 168-198 (55.4 - 65.3%) -- guaranteed 2HKO after Leftovers recovery
0 SpA Mega Glalie Freeze-Dry vs. 0 HP / 0 SpD Keldeo: 194-230 (60 - 71.2%) -- guaranteed 2HKO after Leftovers recovery
+1 252+ Atk Mega Glalie Earthquake vs. 0 HP / 4 Def Mega Metagross: 238-282 (79 - 93.6%) -- guaranteed 2HKO
As for
Floatzel, my big issue with it is that I can't think fo any real chance it would get to set up that BD. Its bulk is pathetic, even with a good speed tier, and unlike Azumarill, its offenses unboosted are not high enough to threaten anything but a SE target out, especially lacking a secondary STAB. People also note the ability to pass that boost, but unless Floatzel passes that relatively quickly, it's not always going to pan out. Odds are if something is out that can cripple a +6 Floatzel with that speed, even if he BP's, the switch-in could be just as vulnerable to the crippling attempt, be it a Phazer, TW (no one would try a burn on him, so switch doesn't have that to fear), some bulky resisting wall, or just a faster Scarfer.
0 SpA Mega Venusaur Giga Drain vs. 0 HP / 0 SpD Floatzel: 332-392 (106.7 - 126%) -- guaranteed OHKO
+6 252 Atk Floatzel Return vs. 252 HP / 240+ Def Mega Venusaur: 240-283 (65.9 - 77.7%) -- guaranteed 2HKO
+6 252 Atk Floatzel Waterfall vs. 252 HP / 88+ Def Ferrothorn: 150-177 (42.6 - 50.2%) -- guaranteed 3HKO after Leftovers recovery
252 Atk Floatzel Waterfall vs. 252 HP / 252+ Def Unaware Clefable: 100-118 (25.3 - 29.9%) -- 0.1% chance to 4HKO after Leftovers recovery
Yeah, maybe there's not much that can switch in, but this is something that needs 50% of its health to Boost relying on 85/55/50 bulk or 105 offense to threaten something out. With how the metagame is shifting into bulky offense, not to mention powerful tanks and resistors like our own recently added Mega Agrron (if other Theorymons are to consider with the ladder now), finding something to beat him before he boosts, or even just tank a hit and revenge after the boost isn't as tall an order as it would seem.
+6 252 Atk Floatzel Ice Punch vs. 252 HP / 16+ Def Mega Aggron: 121-143 (35.1 - 41.5%) -- guaranteed 3HKO (Steel/Dragon)
+6 252 Atk Floatzel Waterfall vs. 252 HP / 16+ Def Mega Aggron: 195-229 (56.6 - 66.5%) -- guaranteed 2HKO (Steel)