Palm OS is actually being discontinued, no future releases. Even Palm is moving off of it.

I have used the following, Blackberry's OS, Windows Mobile, Windows CE and Symbian.

My favorite so far is windows mobile as it can be very open and very powerful. Windoes CE is the most open, however it literally crashes. Windows Mobile has never crashed on me. PC integration is obviously top notch. I adore my blackjack, not such a fan of the blackberry my company gave me.