Hand soap or dish washing soap as you did not mention what type of sink.

Kitchen / bird bath (Bradly)or a basin also some soap dispersers use powder like Borax others use liquid.

Then you have to give an idea of what what the type of dirt / grease hand washing etc this "sink" is used for

Laundry room slop sink / sink in a garage would require a soap with pumice

kitchen sink / medical office require anti fungal soaps.

Not one "soap" fits all and sink can mean a variety of applications.

If your talking domestic kitchen sink look for any liquid on sale or read the dispensing directions as some require the soap to be diluted so it will pump easier as the viscosity maybe to thick.

I just noticed it said liquid so

I Just asked the expert she suggests Dawn or if you cant get Dawn get Ajax

Full strength unless you get the concentrated one then it has to be diluted so the pump can dispense it.

Do not forget not to fill the bottle to the top because once you place the plunger in ther excess will make a heck of a mess