Everyone here pretty much agrees that the
Socket Low Power CF Card is the best..........it's also the most expensive.
However, you can get the "same card" when you buy a
Symbol, it is the same card as the Socket, but is just rebranded. It comes with it's own drivers, but they should be interchangable with Socket's should you have any problems.
I've got a Symbol on an X5 Advanced running PC2002 (A04), and it was a very simple set-up using Symbol's Drivers and I have not had a single problem with the card. Good range and power saving.
I also remember reading that the
Ambicom CF Card is also a "Socket", but I cannot verify that.