My vote was for Roth because of the depth of his treachery. Roth engineered the Tahoe hit in Michael's bedroom with the help of Fredo. Roth directed the attempted hit on Frankie Five Angels to make it appear as if Michael was behind it. This prompted Frankie to turn against Michael and cop a plea with the government.

In effect, these two plots by Roth led to Michael losing his family. Once Michael learned who the traitor was, Fredo was killed. All of the treachery & deception led to Michael losing the trust of his other (non-blood) brother, Tom Hagen, and his wife, Kay.

I believe that Michael's life unraveled in GFII mostly because of the actions & manipulations of Roth.