It’s a tie between Mario the Pious and Cardinal Bernardin. In the mid-1980s, they tag-teamed coming up with demonic rationalizations for supporting abortion.
Cardinal “seamless garment” Bernardin was certainly an extremely effective heretic. Probably the single worst bishop in the American Church, since he managed to hide his true colors better than most of the other heretics, while starring in the press as their favorite go-to.