Currently the best Flashcard you can get is the CycloDS Evolution, which supports realtime save, ingame menu as well as battery saver, Wii connectivity and other nice goddies I can't think of.
It is quite pricey, but if you were to buy it you won't be disappointed.
The next best cards are the Acekard 2.1, EDGE, Acekard2i and M3 Simply (I think), Acekard 2.1/i have a great third party firmware team, cheap prices, nice menu as well as having some of the features of the CycloDS minus stuff like realtime save.
The Acekard2i is the first flashcard to work on the Nintendo DSi, it has the same functions as the 2.1 version.
The EDGE is clone card f the CycloDS Evolution with less features and I haven't bothered to do research on the M3.
If you want my opinion, if you only want to play games, then get the Acekard 2.1 or 2i (if you are getting a Nintendo DSi) as it has great features and an alright build for $12 dollers, but if you want the best and don't care about prices, then get the CycloDS Evolution.
Be sure if you want to play GBA games, then get the EZ Flash 3 in 1 Expansion Pack with the flashcard of your choice and DON'T FORGET A MICROSD CARD lol, all of this can be found on DealExtreme.
Oh and never EVER get the R4, not only is the firmware team dead, but none of the cards supports SDHC MicroSD and of course there are many fakes running around.