I see the good sense in chronological order for sources and often that would be the best guideline.
In the Puritan Great Migration profiles I put Robert Charles Anderson's work at the top, because it's recommended by the PGM project. After that go birth, marriage, and death records from state Vital Records and church records, in that order. Then comes personal diaries from the lifetime of the ancestor - Dr. Winthrop's medical notes or Gov Bradford's diary, for example. Also inserted in the right chronological order with Vitals and church records go any immigration records created at the time of the journey. This includes notes from church records and court records on migrations to new towns, Freeman's Oaths, Military service, jury duty, etc.
Land records could be inserted chronologically, but I prefer to keep them together, right after the Vital Records section - as far as listing sources is concerned. In the biography text, they would be placed chronologically.
Put simply, any primary records created in the lifetime of the ancestor go at the top of sources in chronological order, with Wills placed after the death record. Although Wills are created before the date of death, they are not put into effect until after death, burial, inventory of estate, and probate.
Like Great Migration Begins for PGM project profiles, the profiles designated by the Magna Carta Project have a designated primary source, which is Magna Carta Ancestry: A Study in Colonial and Medieval Families, by Douglas Richardson, which is placed at the top of the source list. His newer series, Royal Ancestry: A Study in Colonial and Medieval Families, goes next, and after that Medieval Lands, which is one of the designated sources for the European Aristocrats Project, which often overlaps with the Magna Carta Project.
Anderson and Richardson cite and quote primary sources throughout their extensive works, which is why they're chosen for the appropriate projects and why they should go at the top of the source list.
Articles by certified genealogists published in The American Genealogist, the New England Historic Genealogical Register and other such publications go right after Anderson, Richardson, primary records, and Medieval Lands, because they cite and usually reproduce primary records, and also apply good genealogical science in their interpretations of events.
After that are collected histories and lineages that cite sources, the best of which reproduce wills, etc.
Find A Grave Memorials go after the other primary sources - it is a primary source for the death date, not for the birth date, best with an image of the gravestone. Some FaG memorials quote obituaries, adding valuable family information.
Ancestral File, Ancestry Trees, OneWorldTree, are trees usually without any sources, therefore they are questionable. If they cite a primary source, it should be located and added as a source. I put them at the end of the source list.
This list is something I've developed over time, and not all the profiles where I'm PM have been done in that order. I think it is a good idea for the community to agree on how we will order the source list, and I would like to hear from more of our members on how they order source lists.
It is an important question, especially when working on a profile with many merges, and putting the sources together.