The Frag Grenade wins hands down but in second place comes the smoke grenades, the incendiary and ink grenades are dam useless unless you either tag an enemy with it or hit them directly with it.
Frag Grenades> Smoke Grenades> Incendiary Grenades> Ink grenades
WTF is the Ink grenades for other than to place some tiny ass black cloud of smoke that does nothing!