Gmail/Google Calendar or Outlook.com are probably your two best options.
However, in my experience there are some issues with Outlook.com on Android, and issues with Google contacts and calendar on Windows 8.
I don't have any issues with either Google or Outlook.com on iOS.
I wrote up the issues I've had with Outlook.com on Android here:
http://forums.androidcentral.com/android-4-4-kitkat/385091-syncing-outlook-com-android-4-4-a.html
However, I no longer use Outlook.com and these tests are a few months old, so things might have improved since then.
Gmail and Google Calendar? Works for me on Android, WP, Win 8, iOS and Mac.
My experience with Google on Windows 8.1 is different. Gmail is supported well enough. But there is no native support for Gmail Calendar, and only partial support for Google Contacts.
Google contacts is accessed by connecting your Google account to your Microsoft account. But this only provides read-only access to your Google contacts from within the Windows 8.1 People app. Google Calendar is not supported in the native Windows 8.1 Calendar app. However, there are 3rd party apps in the Windows Store which provide Google Calendar support (e.g. Gmail Calendar, OneCalendar).
I expect Microsoft will provide full native support for Google Contacts and Calendar in Windows 9 (since full support was added to Windows Phone fairly recently).
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EDIT: Just noticed in your original post that you don't used Android. So the limitations I mentioned of Android for Outlook.com probably not relevant to you. However, I've left them in this post because they are relevant to the title of the thread. In your case (non-Android user), Outlook.com is probably best.