I have the G4 Elegance and its very powerful. I also have the battery operated version of the Big Boss (it was sold only by one company years ago) and it is strong as hell, and it keeps on going.
If you're looking for power, these are just below Hitachi or Wahl strength. Few other vibrators come close. JoyDivision makes some strong vibrators, but they are hard to get in the US. I'd go with Fun Factory, which like JoyDivision, is also a German company.