i would say that the strength of a weed depends on 2 key things:
1) the strain obviously, been bred and crossed lots of times by the wizards of holland breeding the stongest and most prominant male and females out a hugeeee batch of plants, so strains like power plant( very nice) northern lights is strong and tastes amazing!

and

2) the time, devotion, experience and the grow setup of the person growing a strain u cud give a newbie grower the worlds most strongest skunk seed and little knowledge on growing and that strain will not have the chance to perform to it full potential. give a average seed to a pro grower and that strain will be pushed to grow its best and produce large fruity strong stinky bud!