Depends on what you need. They each have their advantages and disadvantages. Also depends on your carrier and what they offer.
As the previous poster says, the Palm OS is not the greatest, somewhat dated and unstable. IMHO, the Treo line suffers from huge quality issues and are prone frequent to reboots. I had 4 before i gave up.
Windows Mobile is newer technology, but still somewhat unstable and prone to reboots as well. Lots of 3rd party software for WM devices. You can set up email to get it every 15 mins or so, but battery life will go down in a hurry.
The BB is bulletproof, but is oriented to business users, but the Pearl and 8800 seems to be more in the consumer line. Great for email, adequate as a phone and PIM.
The 71xx line has been around for awhile and basically replaced by the Pearl, the 87xx and now the 88xx series. I suggest you take a look at the features of each and what you need in a phone. Remember that you will need a BB dataplan for Internet and Email. Nothing else will work.