Well,actually depends on Pokemon but here is my answer anyways:
Groudon-Primal (Strongest and the most popular Ground-Type) can be OHKO'd almost all the time by a cloud nine golduck.(yep,a freaking golduck.) Choice scarf Modest nature with surf is all that you need.
Arceus-Ground (just after groudon in the matter of raw power) a water spout at full health would be a really good choice.(scarf blastoise will always outspeed it and will pack a huge punch likely 2 hit KO'ing it.)
Zygarde All Forms Ice beam.
Garchomp Ice beam
Landorus both forms Ice beam.
Unless the Pokemon we are talking about is Primal Groudon,a scarf blastoise will outspeed EVERY SINGLE ground type (with scarf and nature doesn't matter unless it lowers speed.)and either water spout or Ice beam it to death.Also if your blastoise is at low health,you can run Hydro Pump along with water spout to take advantage of your low HP with torrent doing just about the same damage.