Tallest ever semi-seriously proposed in Phoenix was in the 1980s by Georges Schriqui--the "Phoenix Tower" would have been 114 stories and the tallest building in the world at the time at 1691 ft.
Never got seriously off the ground, as it was ruled a flight hazard by the FAA and talks with the City broke down over them wanting the developer to include City infrastructure improvements.
There have been a number of other proposals over the years more in the range of 600+ feet, but this is the tallest.