This depends on where the weeds are & what you want to do with the area after the weeds are gone.
For paths, drives, paved & gravel areas the best thing is sodium chlorate. A bit old fashioned now it is a permanent weedkiller that poisons the ground/ soil for plants with sodium, also available in salt & bleach at lower concentrations. It can be washed off paving on to other areas by rain so only use where appropriate. I do not follow the instructions on the packet 'cos the results that way are pants! I sprinkle the neat granules where the weeds are growing i.e. between house & path. If you do this just before rain you will save yourself the job of lightly watering it in or you can just let nature take it's course. Stand back & watch the b*gg*rs die! Re apply when you see signs of invasion! Should be good for at least a year (sometimes 2) unless you have a very lot of weeds,lol.