The ABC priority code was associated with the DayTimer paper organizer, but more widely taught by Franklin, later Franklin-Covey. A represents something that must be done today, B something that should be done today, and C something that could be done today. The approach taught was to make a daily list, and assign A, B or C to each item and then a numeric code representing an order: A1, then A2, then B1, et cetera. This was to be done every day, and then traversed in order.
Contrast this with the more fluid approach of GTD. There is a clear distinction between things that must be done today and everything else. The use of a star or flag to mark priority items, or equivalently, the use of a hotlist, is optional.
I have seen a fair number of people proclaim that GTD doesn't work. Almost always they have misunderstood some key idea, usually substituting something more complicated or silly, e.g. "Every next action must be associated with a project" or "Each project can have only one next action." These are often called straw man arguments in rhetoric and logic: attacking a non-issue.
I do use a star/flag because the list software I use supports it, and I find it helpful. Anything more would be too much for me.