I'm still looking for Labyrinth and I still don't know if Pin Ups and Tonight are worth getting. Or Tin Machine II.
I think I have most of his albums by now. I wish I knew which are still worth buying...
If you've got Never Let Me Down, you might as well go for Tonight, too. I'd recommend holding out for the Virgin rerelease with the bonus tracks, since This Is Not America is a favourite of mine and a lot of other people seem to like Absolute Beginners. As The World Falls Down would also give you a taste for one of the better songs from Labyrinth.
If you really like the sound of Ziggy Stardust, you're more likely to enjoy Pinups but a lot of people see it as his least essential release. Tin Machine II you should definitely get, though, if for no other reason that its scarcity. At any rate, it really is a lot better than Tin Machine's first effort.