Let's be fair to Apple Reminders first, because it deserves it: it's free, it's already on your iPhone, Siri can add to it hands-free, Smart Lists are genuinely useful, and since the iOS 17-era overhaul it's a properly good task app. For someone fully inside Apple's world, Reminders is often all they need, and there's no shame in that. This is not a “we beat Reminders” page.
The reason you're looking for a cross-platform Apple Reminders alternative is almost always one specific thing: Reminders is Apple-only. No Android app. No real Windows app. The web version is limited and tied to iCloud. So the moment your life isn't 100% Apple — an Android phone, a Windows work laptop, a partner who isn't on iPhone, a shared list someone on Android needs to see — Reminders quietly stops being the answer.