My vote is on SOT-23-6 or -363 devices. I have seen surprising consistency in pinouts, with the most common being
1 = BST, 2 = GND, 3 = FB, 4 = EN, 5 = VIN, 6 = SW. Examples: MP2456, LMR12007, RT7285, etc.
Possibly the next most common is
1 = GND, 2 = SW, 3 = VIN, 4 = FB, 5 = EN, 6 = BST, e.g. AP62150, TPS56120x, etc.
A couple other pinouts are much less common and can be safely ignored for these purposes.
Obviously, ratings and characteristics vary across all of these, but some are drop-in replacements for each other, and some can be adapted with other component changes. Buck regulators up to 60V 3A or thereabouts are available in this package; impressive ratings.
The next most common packages (say, SOIC-8, SON-8 or 10, MSOP-8, etc.) may show some promise, but I just haven't looked at them personally in much detail, so I don't know how general they are. There may also be a lot of parts that have similar or identical footprints within a family, or between families from the same manufacturer, which may still be of some merit -- say to avoid low stock of one part, if not the ability to shift supplier entirely.
There's also something to be said for old and generic types, like MC34063, as terrible as they are. Dating back to the days of second-source rules (as dictated by... defense customers, I think it was?), many still make the part. It does perform quite poorly (low efficiency; low frequency --> large capacitors and inductor required), but it may well be the case that that is an acceptable compromise for some applications.
Even further, one might consider discrete circuits (whether including generic functions like op-amps, comparators, logic ICs, or wholly transistor-level), but these do come at significant cost in design work, range of ratings, performance, absent features, and large layout area / parts count / assembly cost.