I have an Ipod Touch, and an Ipad. I also have had the HTC Hero, the Motorola Milestone (droid 1), Samsung Galaxy, Droid Pro, and Milestone x (droid X). Still have the last one (as well as another device)
A few months ago I would have told you Android, hands down. Loved the customization, liked that I could load different roms, tweak all the screens, the widgets, all of it. All to often over the time of having these phones though, I would run into problems with apps. Its available for this one, but not that one. It is available with this provider, but not with this one unless you root and take a chance with loading it yourself.
I started to get tired of retweaking every time I changed roms. It simply wasnt for me anymore.
IOS would have been at the top of my list, but my provider did not offer it, so I looked at WP7 as an alternative.
Ive already posted my review of that in this section, safe to say that I am extremely pleased with it.
So as to what I prefer? Right now, for a phone OS, I say WP7. For a tablet, because I have no experience with a windows tablet (although the new ones look promising) it is iOS hands down. The Ipad is amazing.
For the tinkerer in me, android still is fun from time to time. Once they get all of the hardware and software more in line across the board (and what providers will let you do what with it) I may give it another try.