Hmm... Nokia N series can't be called as a "pda phone", it's more of a multimedia phone.
MY choice:
1. O2 XDA mini - it got a hell lot of features plus the pda features.
2. Nokia 9500 - It too is a good phone but a bit less on pda functions.
Check out my Review here:
http://www.thinkdigit.com/forum/viewtopic.php?t=12807
3. Nokia 7710 - Less features compared to the above phones but costs less too
4. Palm one's Treo 600
5. SE's P910i
But my personal choice would be O2 or nokia 9500.