It should be the Scottish Public Services Ombudsman following Stage 1 and Stage 2 complaints completed with the public body in question.
But good luck with that one because of Covid, there is a minimum of a year's backlog before they can even start looking at a case, and they are a fairly toothless version compared to the English one (which isn't brilliant but will at least respond in an intelligent and non-robotic manner). Good luck with getting anything other than a version of "computer says no" from the Scottish one. This is what the Ombudsman had to say to the Scottish Parliament about the backlog:
"At the same time—this is not linked to lockdown, as we were doing it anyway—we have started to take a resolutions approach much more. If we get a complaint and can see a way of resolving the issue without investigating it, we treat it slightly differently now. For example, housing complaints about repairs lend themselves to someone simply picking up the phone and saying, “What would solve this for you?” Going to the public body would solve the issue. We increasingly take that approach, and we have issued guidance to public bodies about trying to take that approach."
A bit concerning that theres no robust and consistent procedure for dealing with complaints, and the dealing of your complaint depends on someone being able to pick up the phone in another organisation and have a fruitful chat with them. In other words, its all "who knows who and can have a little word in their ear", rather than open and transparent governance.