Cloud sync, tagging, due dates, projects, focus areas, notes, recurring tasks. I've been using Things for years and love it. Might have slightly fewer features than you need, but I think most of these other ones have too many features you don't need (like team collaboration). If you need something slightly more robust than Things, I'd try OmniFocus (which I will recommend separately). Pay once for the apps (Mac and iOS), and cloud syncing is free. No subscription fees.