I hadn't read every post here,if it is discussed already please note me of that,thx.
I suppose Choice Spec Magnezone to be A-.
Though can be a check against Serperior and Thundurus,safely trapping Mega Scizor,it lacks power to breakthrough bulky Ground- and Grass-Type,and Ground-Type can block the Volt Switch to prevent it from making any contribution after your trapping job - if your opponent did have a Steel-Type for you to trap.
Though still cannot surpass the Water/Ground Pokemons and the fat Chansey,a Spec Magnezone can kill almost any fat-mons if predicted correctly on their switch-in,dealing decent switching pressure to Balance and Semi-stall teams.
Spec Magnezone also checks CM Slowbro and CM Clefable,while come down to the Scarf Magnezone,it checks the former only when has 60% more health itself and would be killed back when the latter runs Flamethrower.This is an advantage of the Spec set over the Scarf set.
252 SpA Magnezone Flash Cannon vs. 252 HP / 76 SpD Garchomp: 138-163 (32.8 - 38.8%) -- 99.3% chance to 3HKO
252 SpA Magnezone Flash Cannon vs. 252 HP / 8 SpD Landorus-T: 156-184 (40.8 - 48.1%) -- guaranteed 3HKO after Leftovers recovery
252 SpA Magnezone Flash Cannon vs. 252 HP / 112 SpD Hippowdon: 148-175 (35.2 - 41.6%) -- 80% chance to 3HKO after Leftovers recovery
252 SpA Magnezone Flash Cannon vs. 252 HP / 0 SpD Mega Venusaur: 112-133 (30.7 - 36.5%) -- 64.5% chance to 3HKO
252 SpA Magnezone Flash Cannon vs. 252 HP / 84 SpD Celebi: 120-142 (29.7 - 35.1%) -- guaranteed 4HKO after Leftovers recovery
252 SpA Magnezone Flash Cannon vs. 252 HP / 120 SpD Amoonguss: 136-162 (31.4 - 37.5%) -- 0% chance to 3HKO after Black Sludge recovery
252 SpA Magnezone Thunderbolt vs. +1 252 HP / 24 SpD Slowbro: 230-272 (58.3 - 69%) -- guaranteed 2HKO after Leftovers recovery
+1 0 SpA Slowbro Scald vs. 0 HP / 0 SpD Magnezone: 142-168 (50.5 - 59.7%) -- guaranteed 2HKO
252 SpA Magnezone Flash Cannon vs. +1 252 HP / 84+ SpD Clefable: 158-188 (40.1 - 47.7%) -- guaranteed 3HKO after Leftovers recovery
+1 0 SpA Clefable Flamethrower vs. 0 HP / 0 SpD Magnezone: 204-240 (72.5 - 85.4%) -- guaranteed 2HKO
And,just compare with Hoopa-U and Keldeo.
Keldeo
Choice Specs: S
Choice Scarf: A+
Life Orb: A
Calm Mind: A
Hoopa-Unbound
Life Orb: A+
Choice Band: A+
Choice Scarf: A
Assault Vest: A-
Both of them have a higher viability ranking on their Spec/Band sets than their Scarf sets.Though Keldeo do runs Spec more often than Scarf,Hoopa-U has more Scarf sets on Usage statics,when CB set has a higher viability.
As with the Magnezone,Scarf set has 45% usage and Specs is 30%.
The Specs Magnezone,IMO,is underrated,should be A-.
BTW,the biggest threat against Magnezone,or a Offensive team with a Magnezone,is Zard-X.It can take advantage from the dead of trapped Steel-Type Pokemon,and tear your team with a single Dragon Dance,or weaken your check with a random hit - many Zard-X checks are too shaky to be weakened.