From what I've read and seen about the USA, there was a period the Outlaws and Pagans seemed to be absolutely out of control and ruthless. Mongols, Bandidos and Sons Of Silence were/are up there as well.

In Europe in the Netherlands and Northern Germany the Hells Angels can be extremely dangerous, but they're generally pretty sneaky about it. Two cases come to mind: in Holland the Limburg-based Nomads chapter of the Hells Angels once tortured a Colombian drug trafficker to death, cutting his head off, chopping up his body and dispoosing of him after stealing his narcotics. This case lead to settling of internal disputes in the HA Holland.
In Kiel, Northern Germany, they discovered a torture chamber owned by the Angels. A rival drug trafficker was found slaughtered in that chamber. These sets can be pretty bad.
In Trois Rivières and especially British Columbia the Angels can be just as brutal, the Vernon Greeks case proved that.

The other main European club Satudarah has been linked to vicious cases as well. They were linked to the "Tattoo Killers", a crew of contract killers of mostly Moluccan descent that were involve in an array of murder and maimings for hire