what's your budget and what are your preferences/requirements for OS and features?
also, are you looking for performance or compactness and are you willing to sacrifice one for the other?
If money's no object, the OQO Model 02 is as good as it gets:
www.oqo.com/products/index.html
The only PDAs I've used were Palm OS. My two favs were both Sony Clies. The only shortcoming I see with Palm is that it can't compete with the functionality of a handheld running Windows CE. I think if I went with a pcoket PC I'd probably use it more. To be honest the last PDA I bought just collects dust.
I bought it when it first came out and it made a neat toy for a while but that was about it...
www.amazon.com/Sony-PEG-TJ27-U-CLIE-Handheld/dp/B0001E75LC