Unaware Clefable is truly amazing. It walls many pokes like Azumarill (both Banded and Belly Drum), Mega Medicham, Thundurus, Garchomp (w/o Poison Jab), Conkeldure, Keldeo, Mega Pinsir, Talonflame, Tyranitar... all are dangerous OU threats and Clefable always win one vs one. Just max HP/DEF+ and give it Wish/Protect/Moonblast. The last move slot can be either Calm Mind (after a Calm Mind, Flamethrower Char Y in sun fails to 2HKO it) or Flamethower (to surprise switched-in Scizor and Ferrothorn).
Unaware Clefable does not wall Banded Azumarill, but neither does Quagsire for that matter. Unless the calculator is broken:
252+ Atk Choice Band Huge Power Azumarill Play Rough vs. 252 HP / 252+ Def Unaware Clefable: 235-277 (59.6 - 70.3%) -- guaranteed 2HKO after Leftovers recovery
(235, 238, 240, 243, 246, 249, 252, 255, 258, 259, 262, 265, 268, 271, 274, 277)
252+ Atk Choice Band Huge Power Azumarill Play Rough vs. 252 HP / 252+ Def Unaware Quagsire: 214-253 (54.3 - 64.2%) -- guaranteed 2HKO after Leftovers recovery
(214, 217, 220, 222, 225, 228, 229, 232, 235, 237, 240, 243, 244, 247, 250, 253).
Neither of them can wall Mega Pinsir either:
252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 252+ Def Unaware Clefable: 217-256 (55 - 64.9%) -- guaranteed 2HKO after Leftovers recovery
(217, 220, 222, 225, 228, 229, 232, 235, 238, 240, 243, 246, 247, 250, 253, 256)
252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 252+ Def Unaware Quagsire: 198-234 (50.2 - 59.3%) -- 79.3% chance to 2HKO after Leftovers recovery
(198, 201, 202, 205, 207, 210, 211, 214, 217, 219, 222, 223, 226, 228, 231, 234)
That said, they're both good. I think Clefable can be more useful in some cases due to being able to run a cleric set while still stopping sweepers.
Something like:
(Clefable) @ Leftovers
Ability: Unaware
EVs: 248 HP / 252 Def / 8 SpD
Bold Nature
- Calm Mind / Thunderwave
- Moonlight
- Moonblast
- Heal Bell
Slashed Thunderwave in case you need to stop something like Mega Pinsir. You can't wall it, but you could come in after a KO, tank a hit, and paralyze it.
Edit: It's probably also worth mentioning that Clefable also takes special hits much better than Quagsire, although that means less in the current (very physical) meta.
252 SpA Alakazam Psychic vs. 248 HP / 8 SpD Clefable: 163-193 (41.4 - 49.1%) -- guaranteed 3HKO after Leftovers recovery
(163, 165, 168, 169, 171, 174, 175, 177, 178, 181, 183, 184, 187, 189, 190, 193)
252 SpA Alakazam Psychic vs. 252 HP / 0 SpD Quagsire: 216-255 (54.8 - 64.7%) -- guaranteed 2HKO after Leftovers recovery
(216, 219, 220, 223, 226, 229, 231, 234, 237, 238, 241, 244, 246, 249, 252, 255)