The Meiri and the Rema, for starters, don’t believe Christianity is avodah zarah. Same applies to the shita of R’ DZ Hoffmann and others. I wouldn’t classify their approach as an extreme minority.
Unitarianism, for certain, wouldn’t be considered avodah zarah by any account (R’ Henkin allows Jews to enter Unitarian churches), and this would apply to other denominations that don’t believe J.C. was a deity. (J. Witnesses, etc.)