Cloud sync, due dates, projects, contexts, notes, recurring tasks, multi-level tasks. I've been using OmniFocus at work for years and love it. Frankly I think most of these other recommendations have too many features you don't need (like team collaboration). If you need something slightly less robust than OmniFocus, I'd try Things (recommended separately). Pay once for the apps (Mac and iOS), and cloud syncing is free. No subscription fees.