IMO you can't go wrong with any currently on the market. They'll all run 99+% of games. Here are some to look at though:
R4i Gold 3DS (r4ids.cn) http://nds-card.com/ProShow.asp?ProID=149 - This one is recommended the most by users here because it has WOOD support, which features a fast, clean menu and many features such as multiple save slots and game language select. Weakest point may be the RTS, only one slot and I haven't heard it being very good.
2016 R4i SDHC Dual-Core (r4isdhc.com.CN) http://nds-card.com/ProShow.asp?ProID=507
This card is very underrated (in my opinion). It supports WOOD just like the above card and can be had for cheaper. The only difference is lack of RTS, which you might not want anyway. (If you would like to buy this from me w/ Paypal please pm )
Built in processor lets you emulate GBA and SNES pretty well. Probably the best savestate support. Menu is pretty good too. Has multiple save slots, save state slots, ingame-cheat menu, slow motion, perhaps more I can't think of right now. Downsides: the premium you have to pay for it, and increased battery drain.