Pretty surprised that a Mega Venusaur set is not posted yet.
Ability: Chlorophyll (Thick Fat when mega evolved)
Modest Nature (+SpA,-Atk)
-Hidden Power Fire/Sleep Powder
With GenVI, Permanent weather is gone, and now Grass types are immune to Sleep Powder. In other words, Chlorophyll Sweeper Venusaur had gotten the nerf stick quite hard this time. But his new Mega Evolution is great, losing his Fire and Ice weakness is cool.
Despite no Leftovers, Synthesis and Giga Drain should provide enough healing for Mega Venusaur to remain healthy, while with the emergence of Fairies, Sludge Bomb is a great move to hit them with. The last slot is for Hidden Power Fire to hit Steel types that otherwise wall this entire moveset. Sleep powder is an option, but running that will leave you helpless against Ferrothorn.