I've had good luck with the Sandisk Ultra II (both versions), set up with WinUAE, or a real Amiga with a boot disk, hdinstools, and PFS3. I have encountered issues with an "Extreme III" 4GB card, but haven't yet figured out whether it's a compatibility issue, or a defective card.
If you run into problems, there's a huge thread at the EAB with lots of good info.