android.
unlike any of the other os, android is open source. the apps are growing out of controll and will soon take over apple (its open source). the fact that its open source means that its not manufacturer specific.... you can have a motorolla android, an htc, samsun, lg....... the os is not locked down, which means that if you like a certain brand, then its likely that they will have a droid.
palm is good, if you are talking about the new palm os. it relys on complete cloud computing, and has a truer linux os than any of the other os...... the os was developed by some of the people that developed the iphone. i would stay away from their earlier models though.... they were the first true pda phone, and rode that wave with out anything knew till the folks from iphone came over to help with the development.
blackberry is....well, blackberry. blackberry is becoming outdated. they havent really come out with anything new or groundbreaking. their adds rely heavily on the bbm (blackberry messenger.....but that only works with other bb devices.) their app world has some nice clean apps, but alot of them are apps you have to pay for, and they dont have half of what the android or the iphone has to offer..... however all that being said, they have a fairly reliable os that runs smoothly, the push email is revolutionary, and is still the buisness mans phone of choice.
i really dont know a lot about symbian, except to know that it is really popular over seas.