Windows Phone, particularly Windows Phone 8.1. It had its flaws, the biggest being the lack of third-party support of course but the UX was the best I've ever experienced on a mobile device.
The UI was very different from Android/iOS but felt more consistent and intuitive. The Live Tiles especially were just such a good feature to get as much information as possible on one screen. And it all looked so incredibly good for the time with its silky smooth animations.
It's a shame Microsoft ruined it all with Windows Phone 10...