Gankers - part of the game and to some POV they add imaginary 'danger' and 'risk', to another persons POV they are tedious as has been pointed out many times only noobs get ganked successfully, experienced players just get away and its just tedious repetitive behaviour
Griefers - toxic to any community and game because they try and destroy the game itself
Pirates - neither of the above and my POV is they add gameplay and an experience, to another POV they hate it and class it as ganking
Problem is it can be quite hard to tell who is what in a given situation.