Maybe Dragonite with Fire Punch and intact Multiscale can counter Venusaur, but even Extremespeed does not OHKO him, while after Dragonite's Multiscale was broken, +2 Sludge Bomb is a clean OHKO on him.
Speaking of Extremespeed, it is not actually extremely powerful. It is weak and is only useful against frail things or things that have been moderately weakened. I do never use Extremespeed, it's appealing but only useful one time or another, if you really fear Weavile and Mamoswine, you are better off using a Fighting-type partner to check them.
Choice Band Scizor will also never OHKO Venusaur, while it is destroyed by HP Fire. Even Ice Shard from a Life Orb Adamant Mamoswine fails to OHKO Venusaur, the same applies to Jolly Weavile.
All these things can however, 2HKO, and if Venusaur was weakened a bit before, it can actually be OHKOed. But still, no form of priority can OHKO a healthy Venusaur, even after Stealth Rock, even those to wich Venusaur is weak, even without investiment in defenses. After some rounds of Life Orb recoil, Venusaur may be OHKOed, but you can never rely on priority to kill him.
Unfortunately Venusaur is a monster under sun, there are few things that can stop him; Venusaur can actually defeat the pink blobs since it is immune to Toxic, Thunder Wave does not impede Venusaur from destroying the two (but can facilitate the revenge kill) and it can simply Growth to +6 until it uses Giga Drain to 2HKO the blobs and recover the Seismic Toss damage.
The only full stop to him is Heatran as it is immune to HP Fire and Sludge Bomb and 4x resists Giga Drain, but even then, you must be sure that Dugtrio/Terrakion/wathever you use to kill Heatran is killed, or that Heatran is using Shed Shell.