My answer would be Mega Venusaur.
The reason why I picked Mega Venusaur over Mega Blastoise is because of tier and usage. Both Pokémon pretty much do the exact same thing, but Venusaur just does it slightly better. Here are some pros and cons for each mon:
Good special attack, to make use of STAB moves like Giga Drain and Sludge Bomb
Very solid base 123 Defense and 120 Special Defense
Extremely useful ability Thick Fat (halves damage of Fire-and-Ice type moves)
Gets good recovery with Synthesis, and can stall with Leech Seed and 100% accurate Toxic
Removes Toxic Spikes upon switching in.
Grass / Poison isn't the best defensive typing
Underwelming low speed stat of only base 80
Needs the Venusaurite to get to this form, so it can't hold a Black Sludge or anything.
Is walled by other stallers (mainly fire types) like Heatran, Chansey, and it OHKO'd by STAB moves from Tapu Lele, Mega Medicham and mainly Mega Alakazam
Very solid base special attack
Good defensive typing, by only having 2 weaknesses.
Is able to remove entry hazards with Rapid Spin, while it keeps your traps on the field
Good bulk overall, base 120 Defense and 115 Special Defense
Mega Launcher ability boosts some strong moves
Gets OHKO'd by Eletric-type threads like Tapu Koko
Has to rely on RestTalk for recovery
Is outclassed by other Rapid Spinners and bulky Water-types like Toxapex
Both Pokémon are really solid in my opinion, but I would recommend using Venusaur in OU still. Mega Blastoise does pretty OK in UU though.
Hope this helped!