Pump sprayer is the tool for spot spraying

There is no best lawn weed killer, overall. Various weeds require various herbicides. If you tell us what weeds you have, we can tell you which herbicides are most effective for each one.

The pump sprayer give you much better control. No sense wasting the herbicide on the grasses.

Personally I dislike hauling out the hose for this and that. A good gallon size pump sprayer is just the ticket. The hose end sprayers work well for treating the entire lawn. Once you've got the weeds at bay though, and a reasonably thick turf, you shouldn't really need it for herbicides. A professional quality pump sprayer will last many years and can be disassembled and repaired, o-rings renewed &c. You can purchase lawn chemicals in concentrate form and mix up any amounts or combinations needed. A small amount of liquid dishsoap in the mix will help the herbicide stick to the leaves. Only the actively growing weeds need to be treated, this saves money and lessens chemical runoff into the environment. Be sure to always have the sprayer itself labeled with the type of chemical used.