Whenever I'm copyediting a historical piece I try to pick a word that carries both the period flavor and the right legal nuance. For everyday historical prose editors often lean toward 'sovereign' as the preferred synonym for monarch: it sounds formal, it implies recognized sovereign authority, and it sits comfortably between being too generic and too ornate. 'Sovereign' works especially well when you're describing the institutional role — the crown as an office or power — rather than the person in a conversational way.
That said, context shifts everything. In a medieval setting I might choose 'liege' or 'liege lord' to highlight fealty and vassal relationships; in a narrative about Byzantine diplomacy I would avoid broad synonyms and use 'emperor' because it's specific. I also steer clear of 'potentate' unless I want a slightly judgmental tone, and I use plain 'ruler' only when I need a neutral, catchall word. Good editing is about matching register to era, and for a default, measured, historically minded voice, 'sovereign' tends to be my go-to — it just feels right on the page.