Windows Phone 8, from its announcement to launch.
Nokia announced two new Lumia phones last night, the Lumia 920 and Lumia 820, with the 920 as the flagship. The Lumia 920 packs an 8.7MP PureView camera, a 4.5-inch 1280x768 PureMotion HD+ display, and Qi wireless charging.
Earlier this morning, Microsoft announced Windows Phone 8 at the Windows Phone Developer Summit. Existing Windows Phone 7.5 Mango users won't be able to upgrade, due to hardware differences. Instead, they'll receive a minor Windows Phone 7.8 update.