Depends on what you want. From my own reading, there are main two types of lawn weedkiller, each with a different set of active ingredients. I think the two known brands are Resolva and weedol.
So the idea is that you use a lawn version of resolva to cover the whole lawn, then later in the year if needs be, use weedol for spot treatment, or you can do it the other way round (weedol for the lawn, resolva for spot removal). This is what I did last year and it seemed to work well.
Both Resolva and Weedol will kill buttercup and dandelion in one hit so the choice is yours. If you've got only a few, just get one as a spray for individual hits. If you've got loads or a large lawn, consider treating the whole lawn. Be careful with dilution rates or you'll kill the whole lawn. Be careful how and where you spray spot treatment as it can affect the grass immediately surrounding the weed (as I found out to my own detriment).