It depends what you would like to do with it and what is your budget. Make a list of requirements that you definately need the pda or tablet to do. If you know that you want to use it for making phone calls, start thinking a smartphone (e.g. treo or blackberry).
Most pda’s have small memory with an expansion card slot for extra storage, because the idea of it is to use it with your laptop or desktop. This is why I finally picked the Palm Lifedrive with its 4gb hard drive and the price was $300. I love it. On the same token, my friend is on her second Dell Axim and loves it because it runs windows mobile and works easily with her home computer.
The only thing is both the Palm Lifedrive and the Dell Axim are no longer in production, but if you do a search, you can still find places that have some left for sale.

right now i think the t-mobile wing. you can do anything and everything with this pda. internet, messaging, music, tv, voice calling,maps, restaurant & movie search, news, weather, etc. I love mine! get online and look at the video for the t-mobile wing, it explains everything, and much cheaper than the i-phone!

I PHONE DEFINITELY

check it out