Maemo/Meego. I know there is Sailfish still around, but things would had been very different today if Nokia had put all its weight on it back then.
They should have partnered not only with Intel, but with Palm, RIM or whatever other then-giant to rival Android. Those two went their own ways with WebOS and buying QNX, so maybe they could have agreed to form a consortium for an open and interoperable mobile OS