Nunu has the easiest time ganking for lanes that can put out damage on a target. Since his snowball is a very strong slow, and that you can keep it applied often, you can look more at consistent DPS than burst.
Generally speaking, avoid ganking for low damage types, such as pre-6 Nidalees, unless their opponent is way overextended. And avoid ganking post-6 Nidalees or Teemos or Singeds or the like that are generally a pain in the rear to pin down or approach. Again, unless they're way overextended or you feel you could tower dive sucessfully.