Change your preferences here.
120 Base Power with STAB is a pretty big deal no matter what (unless it's from Present...). However, Power Whip has rather poor distribution, so if not for Ferrothorn's existence, it would be pretty rarely seen in any tier. Ferrothorn uses it highly effectively off of its decent 94 base Attack, though. It is powerful even on the typical defensive set, and it is even more monstrous with a Choice Band. Venusaur also has Power Whip, but it is incompatible with Chlorophyll, which is pretty much what gives it its place in OU in the first place.
Outside of Ubers, OU, and VGC, though, Power Whip actually is pretty rare, thanks to the aforementioned poor distribution. Tangrowth is the most notable user of it in UU and RU. It's good for attacking physically on a defensive set, or it could be used on a Swords Dance set for fun times.
Seed Bomb is a less powerful option that is more accurate.
Wood Hammer is an equivalent that has recoil damage as a drawback.