The poll should include all of his albums from Space Oddity up to Scary Monsters. Every album in that period is very good (1969 - 1980), even the cover album Pin-Ups. After that period all are non-contenders IMO, although; I admit 'Let's Dance' is a very well crafted pop album. Last solo Bowie album I bought was 'Tonight' which was very disappointing. Here is the list your poll should have.
I voted for Ziggy Stardust but it was a hard choice because there are so many other albums that are pretty much equally as good. Ziggy Stardust was the first Bowie album I bought though so is the only real reason I voted for it. Buy all the albums on that list and you have the essential Bowie collection, IMO.