Oops, I voted for 3, but I really have 4.
1st) 2GB DSLinker card...great, but no cheat support. Bad homebrew support.
2nd) R4...turned out to be a counterfeit, but hey it worked!
3rd) Supercard SD...for GBA, I soon switched to EZ 3-in-1.
4th) Acekard 2...THE BEST. I'm using it alongside EZ card, and I love it.
I should have gotten the Acekard 2 first, but it was fun to experiment!