Even with great bulk it's weak to Fairy and Priority Ice, I'm on board with moving it up to A+ but if you're suggesting S then in the offensive DD sweeping department it's outclassed by Megazard X's better offensive typing, stats, and ability. It is more of a defensive DD sweeper and should be used as such.
The thing about Mega Altaria right now is that it's eminently unpredictable. All it really needs to sweep is Dragon Dance + fairy STAB, and its remaining two moves can be setup to beat just about anything. When you bring Altaria out, your opponent has no idea if you're running an all-out offensive set, a defensive hybrid, a fully defensive set, or whether it's even running Dragon Dance at all. You don't know if it can pierce your subs with Hyper Voice, whether burning it will actually make it stronger (if it runs Facade), etc. I've even run into a few Altarias that have run Sing in conjunction with DD to anticipate steel switch ins, and it's annoying as hell.
It is in no way outclassed by Megazard X. The two perform go about sweeping in different ways, but are both incredibly deadly when they get there, and not that difficult to set up.
As for fearing ice priority, there isn't a priority ice user in the game that can OHKO Mega Altaria.
244 Atk Life Orb Mamoswine Ice Shard vs. 0 HP / 4 Def Altaria: 156-187 (53.6 - 64.2%) -- guaranteed 2HKO
252 Atk Life Orb Weavile Ice Shard vs. 0 HP / 4 Def Altaria: 151-179 (51.8 - 61.5%) -- guaranteed 2HKO
Weavile fails to even 2HKO if Altaria is running a defensive orientation:
252 Atk Life Orb Weavile Ice Shard vs. 252 HP / 4 Def Altaria: 151-179 (42.6 - 50.5%) -- 0.4% chance to 2HKO
Meanwhile, Altaria more than comfortably OHKOs:
+1 252+ Atk Pixilate Altaria Return vs. 0 HP / 24 Def Weavile: 872-1028 (310.3 - 365.8%) -- guaranteed OHKO
+1 252+ Atk Pixilate Altaria Return vs. 0 HP / 0 Def Mamoswine: 382-451 (106.4 - 125.6%) -- guaranteed OHKO
As for fearing fairy moves, Altaria is able to OHKO every relevant fairy user in OU currently:
+1 252+ Atk Pixilate Altaria Return vs. 228 HP / 0 Def Azumarill: 382-451 (95.9 - 113.3%) -- guaranteed OHKO after Stealth Rock
+1 252+ Atk Pixilate Altaria Return vs. 252 HP / 220+ Def Sylveon: 309-364 (78.4 - 92.3%) -- 37.5% chance to OHKO after Stealth Rock
+1 252+ Atk Pixilate Altaria Return vs. 0 HP / 24 Def Mega Gardevoir: 436-514 (157.4 - 185.5%) -- guaranteed OHKO