Sap Sipper Milktank is one of the more versatile pokemon in the tier and is one of the best normals all around. I believe it should be A ranked.
When you considering it's effectiveness against Amoonguss, it's usage as a lead, it's ability to team support with Heal Bell and Stealth Rock as well as the ability to go Offensive with a Life Orb and reliable recovery as well as awesome speed, it really is a beast of a pokemon and I don't think a weakeness to Sawk is big enough to put it in B ranking.
Especially sharing a ranking with pokemon like Zweilous.
Zweilous has an extremely marginal niche. It has extremely poor versatility and can only really function as an inconsistent tank due to the extremely poor Hustle ability and mediocre defensive typing.