To my knowledge there is no canonical source which firmly specifies either that (a) a single machine has the highest authority over all other Machines, (b) a "council" of multiple Machines wields overall authority, or (c) the Machines' authority is entirely decentralized and every Machine fulfills its purpose but has no other authority.
Purpose is very important to the Machines -- Smith, Ramachandra, the Keymaker, and the Oracle all demonstrate that their purpose is of utmost importance to themselves and other programs. Consequently, I suspect either one Machine's specific purpose is to wield the highest authority or every Machine's authority is dictated solely by its purpose. This would correspond to possibility (a) or (c).
In the event the truth is possibility (a), we can rule out all but one of your suggestions as the highest authority. The supreme Machine authority cannot be merely a program because a program only exists within the Matrix -- the highest Machine authority would have to exist in the Real World as well.
- The Oracle cannot be the supreme Machine authority because she is a program. Evidence for this is that (a) the Architect calls her an "intuitive program" and (b) she has the green code of a program rather than the gold code of a Machine avatar(1) when Smith assimilates her:
The Architect may be more than a mere program since he designed the Matrix (although he could be a computer program initially written to run directly on the Machine mainframe to create the simulation that is the Matrix). Nonetheless, he regards himself somewhat equal in authority to the Oracle -- he says that he is the "father" of the Matrix and she is the "mother". And since the Oracle is not the supreme Machine authority then neither is he.
The Smith virus was originally an Agent of the Matrix and under the overall direction of the Architect and the Oracle (who designed the Path of the One). By the end he may have become as powerful as all the rest of the Machines (at least within the Matrix) but that does not mean he had authority over any other Machines.
That leaves the Deus Ex Machina Neo talked to at 01 as the most likely candidate. Deus Ex Machina is the machine with the authority to make a deal with The One and allows a human to jack into the Matrix. Moreover, his name is Latin for "God out of the Machine" which is strongly indicative that he holds supreme power like a deity.
Another possibility you did not mention is the Source itself, if that is in fact an artificially intelligent Machine. But the Source may simply be a massive computer with no artificial intelligence, in which case it is not the highest authority.
Again, it is also possible that there is no highest authority.
(1) In The Matrix Revolutions green code is seen in the Matrix itself (indicating a program in the Matrix) while gold code is seen in the Real World (indicating Real hardware).