Decent weedkiller?

Bit of a random one guys but can any of the greenfingered members recommend a good strong weedkiller? I’ve loads of thick, stubborn weeds coming up between the paving stones outside and nothing seems to shift them, I’ve tried two different brands from the local garden centre but they’ve been useless. I have a thin wire brush that gets right in the nooks and crannies so I’m going to dedicate Wednesday to some real hard labour and get as deep as I can to scrape all the crap out, but I want something powerful to kill as much of the root as possible.

This isn’t a highly manicured lawn or dotted with fancy flowerbeds, just a big old patch of grass with a concrete path down the middle… I’m not worried about destroying the ecosystem, I just want the weeds dead and gone. :chuncky:

Thanks folks.