AK2, hands down.
It's my 3rd flashcard (after R4 & EZ V) and, by far, my favorite. I found it much easier to use, more customizable and liked the UI way more on AK2 than the others.
After I tried AK2 for a week, I gave my other flashcarts to my cousin. I do regret that a bit, but I was just trying to get him into the DS homebrew scene and he's married AND left-handed, so I felt sorry for him.
I wouldn't mind trying out the Cyclos, but I'm very happy Acekard so I most likely won't be trying one anytime soon.