I've had a few flashcards since I got my DS, here's my opinion.
GBA Movie Player V2 (CF) - I got this for my GBA to play pocketnes and goomba color, and some Ti calculator emu. Also used it to watch anime (converting was a pain though) and play some music. The price was crazy at the time. With a 1 GB CF card I payed something like $100 for it. This was in -05 I think.
It worked fine but it can't load homebrews over 200-something kilobytes. But a firmware hack was released by chishm which allowed it to be used with PassKey or FlashMe to boot into DS mode and launch DS homebrew. Years later came "Maximum Overload Crack" which allowed it to boot DS commercial roms. Not all games worked, no games after 2007 work. Most recent game it could play and I played on it was Pokemon Pearl, but it kept freezing so I had to save often.
M3 Perfect CF - I found my DS on an ebay-like website and this flashcard was included. This was probably the #1 flashcard back in 2006. Everyone had either this one or SuperCard. It was similar to GBA MP but could boot full GBA roms and DS roms. (again with PassKey/FlashMe) King of Slot-2 devices back then I would say. You had to patch the roms but as I remember it support for the card was pretty good. But the quality of the product was not convincing. It started having problems with freezing, even booting to the menu, and kept getting worse and worse. Eventually it died completely.
R4 - I never had this one, but a friend of mine did. Reason I didn't get one myself was that my DS died in 2007 and I didn't get a new one (err, parts to repair my old one I should say) until early 2009. So I never hopped on to the Slot-1 train until way later. As I understand it though, it was really good back then and similar to M3 Perfect CF it was the king of flashcards for a long time. Many people still hold on to it today. The drawback would be that Slot-1 cards can't boot GBA brew/roms so these days you need two flashcards. One for each slot. (if you're still interested in GBA that is)
AceKard 2 - This is the flashcard I have today. I got it for my birthday last year so I've had it for pretty much a year and a couple of days now. I love it. Can't complain. No patching of either games or DLDI patching of homebrew needed. It eats everything I throw at it. And usually no need to wait for updates either. (though I don't play many of the newer games)
The compatibility and features you get for the price.. It's just awesome. Cyclo DS Evo is similar I guess, but costs a bit more. (at least it did a year ago)
(my vote goes to AK2i
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