Arguably the best is a Fujitsu LOOX N 560.
Got one right here. Very nice. And does email via a bluetooth connected GPRS phone too.
The bad news is that Fujitsu have just pulled out of the PDA market.
The good news is that you can pick them up for slightly over £200.
The bad news is that running the CAA VFR charts on a PDA doesn't work well because the screen is too small to show the airspace etc labels. MM and the VFR charts works well on a 7" or bigger screen - look up e.g. Motion LS800.
I would not recommend a PDA for aviation use, for use as the primary GPS. Little fiddly buttons.