From my understanding, when we use the phrase like "at least one of A, B, C and D", it means AD, BD, CD, ABD, BCD etc. i.e. D is mandatory
IMO, it simply means any combination in which (at least) any one of A,B,C,D is present and 'D is mandatory' is not implied. For example, when we say 'at least any one of Doe1, Doe2, Doe3 and Doe4 should do it' does not imply Doe4 must be one of the doers. Any one of them or any group formed by including two or more of them can do the job.
Now coming to the 'question' proper.
It depends on functionality of A and B in the application.
i. "at least one of A and B"
It will indicate either of these two is required to perform the intended function. Also, combination is not excluded.
ii. "at least one of A or B"
It will indicate either of these two is required to perform the intended function and combination of these two is excluded.
Now, selection of phrase will depend on what your elements do and what combination is required. Again, when one of two is to be selected, then 'best embodiment' dictum will dictate the selection and one has to elect which one gives 'the best embodiment'. If both are identical in importance and performance then it is different matter.