...And if you are relying on causing Slash procs, it is best to have no other elemental damage on your weapon, because they might proc instead of the Slash.
But puncture is best against general Grineer. I use the Boltace personally, because it looks awesome. If I am doing a high-level mission, I either take the Corrosive Projection aura or I mod my melee with Radiation damage and my primary with Corrosive.
Radiation damage is really good against heavier Grineer units such as Bombards and Napalms, whilst Puncture or Slash is extremely effective for lower level Grineer units, such as Lancers and Seekers. Corrosive damage reduces enemy armor, like a Corrosive Projection for your weapons, Viral affects health directly and is good for tormenting those Grineer, and Blast damage makes them your puppets with your knockback effects.