Sheer Force is the superior ability for Nidoking because it powers up virtually all its special moves, as they have secondary effects. Life Orb is also the better item to power them up even further; also, Nidoking won't take Life Orb recoil when it uses a Sheer Force-boosted move. And FYI, Nidoking's Sheer Force-boosted Earth Powers outdamage its Earthquakes:
-Sludge Wave
-Earth Power
-Ice Beam
-Superpower/Stealth Rock/Substitute/Thunderbolt
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb
Ability: Sheer Force
Assault Vest is the preferred item on Metagross to increase its special bulk further.
Unless you're using a hail team, you should never consider Blizzard, especially on defensive Pokemon like Suicune. Here are better takes on its Calm Mind sets:
-Calm Mind
-Scald
-Rest
-Sleep Talk
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
This is the set that's served Suicune well since Gen III.
For Mega Ampharos, Agility is pointless unless it has significant Speed investment, which it lacks. It should stick to either defensive or offensive, not both:
-Thunderbolt
-Dragon Pulse
-Focus Blast
-Volt Switch/Heal Bell
Nature: Modest
EVs: 132 HP/252 Atk/124 Spe
Item: Ampharosite
Ability: Static
or
-Rest
-Sleep Talk
-Volt Switch
-Dragon Pulse
Nature: Bold
EVs: 248 HP/252 Def/8 SDef
Item: Ampharosite
Ability: Static
Just use Hammer Arm on Chesnaught. Also, Bulletproof is superior to Overgrow, since it makes Chesnaught immune to things like Shadow Ball and Sludge Bomb:
-Leech Seed
-Synthesis/Spiky Shield
-Spikes
-Hammer Arm
Nature: Impish
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
Ability: Bulletproof