Snorlax, agreed, forgot about its signature move, except the Signature S-Crystal only works with that move, you cannot use it with the healing Z-Moves, or Z-boosting moves. So I stick with Seld Destruct.
Gyarados @ Flyinium Z
EVs: 252 Atk / 4 SpD / 252 Spe
- Happy Hour
Z-Happy Hour is a upgraded DD that boosts Defenses, and Z-Spite lets you heal off chip damage. After STAB, Supersonic Sky-Strike hits 270 BP, meaning a resisted Sky-Strike is stronger than a neutral EQ or Waterfall.
Snorlax's best option would be Giga Impact + Snorlaxium-Z. Normalim-Z SelfDestruct is still 200 BP, where as Pulverizing Pancake is 210 BP.
I think Special/Mixed Veno has more potential. In the sun, Z-Growth gives +3 Sp Attack. I'd run something like Firium-Z with Earthquake, Solar Beam, Growth, Sludge Bomb / Sunny Day.
Venusaur, I still think pure physical is safer so you won't have to split EVs, and you can avoid weather changes slowing down Solar Beam for Grass-type STAB coverage.
I agree Growth might be better to boost Attack off of, but the other boosts Defenses and Speed as well.