A systemic one.
There are a few around and it will state on the container whether it is or not.
There's a glyphosate based one i use and that one is.

I needed to kill some dandelions that were inbetween a lot of irises and it worked a treat - the irises weren't touched at all.

All this of course as long as you protect the surrounding plants when you apply the weedkiller !
I use newspaper because i find it easiest.

If you use a spray such as Round Up and are very careful only to spray the leaves you want to kill then its OK. Or paint on the weedkiller instead.

Just remember that as far as weedkiller is concerned weeds are plants and plants are weeds, so you must prevent it coming into contact with plants you want to keep. Even a very small amount of spray is enough to damage your wanted plants, which is why if you do spray you should always do it on a day with no breath of a breeze. For the weeds that have grown through your wanted plants painting it on is the best way, but even then be careful not to let any drips fall..