does anyone have a flash card for ds?

Frankly, I believe it depends more on how much you're willing to spend than exactly which one is best. After all, looking at it from a personal standpoint, what do you want the cart for? Playing backup games? Using homebrew? Something else? M3s, R4s, all that stuff, they all do the job if all that you want is playing games. Even if a game doesn't work it'll eventually get a patch or some cheat code that'll bypass protections and whatnot and the game will be running, worse case scenario the online mode not working, but who cares. But, if you don't mind to bang a few more bucks, then you could try to purchase a CycloDS Evolution which is probably the very best cart out there, particularly when it comes to gimmicks. Increased battery life, in-game menu, ability to read text files while inside a game (good for when you want to read a walk-through or a item list or something without having to resort to a computer or paper sheet), supports homebrew and auto-patches just about anything not needing you to do DLDI patches before copying the files. The CycloDS manufacturers are constantly putting out updates to the cart's firmware too, so new capabilities may be added along the way or have improvements of all sorts, plus they typically fix all games with errors or protections, making sure that you don't need to be downloading pre-patched games and stuff.