Well, as long as you speak only about the field of view - then it must be the Voigtlaender 12/5.6 - I think the widest rectilinear (non fish eye) lens ever made (or at least one of the widest).
Now if you are interested in particular format or camera type, things get different. For MF it could be Mamiya 7 with the 43mm lens, for panoramic maybe the 30/5.6 for xpan, for 4x5" probably the Super Angulon XL 47/5.6.
Then there are those rotating panoramic cameras I know little about that go up to 360 degrees (do they?)
EDIT: I just realized (sorry) that you probably meant fixed lens RF cameras. What about the Fuji G690 with 50/5.6 lens (I know it is exchangeable lens camera, but it is very similar to the later, fixed lens GSW690, which has 65/5.6 lens)