Very difficult to say after a decade or so.... I'm discounting the high summit cairns since the summits do it for me anyway, cairned or not.
Strangely one site which (for me) has it all, is one visited only this week.... the moorland circle of Crugian Bach beneath Y Gamriw, near Rhayader. Difficult to find, not on any map and therefore almost certainly with solitude guaranteed. Great cairns surmount the skyline, red kites soar overhead. Bliss.