0 SpA Venusaur Sludge Bomb vs. 0 HP / 0 SpD Sceptile: 116-140 (80 - 96.5%) -- guaranteed 2HKO
From the smogon definition of counter: Pokémon A counters Pokémon B if Pokémon A can manually switch into Pokémon B and still win every time, even under the worst case scenario, without factoring in hax.
About the original Venusaur topic:
I think Venusaur is definitely a S+ tier pokemon in our current OU metagame, I've been using a Calm (252HP\252Spdef) Growth \ Synthesis \ Giga Drain \ HP Fire set recently and it absolutely destroys stall teams, being able to defeat prominent defensive threats like Milotic, Porygon2, Umbreon, Weezing, (most weezings without haze become free setup fodder for this venusaur set, thanks to synthesis), Skarmory, Forretress and Slowbro.
Even fire types that would resist both of it's offensive moves don't really want to switch into a STAB sludge bomb.
Considering other venusaur sets, the best counters I can find think of would be a defensive Gardevoir, Tentacruel (though it can't really do much against defensive venus and loses to earthquake sets), a bulky taunt Gyarados, a specially defensive Arcanine, Crobat or Espeon. Alakazam also forces it out, because no matter the Venusaur set, it doesn't want to eat a STAB Psychic from zam.
To conclude, I don't really think Venusaur is banworthy because of how hard it beats stall, but because of how versatile and efficient it can be on nearly every team.