Best... tough call.
It would probably come down to a fight between the M3 perfect microSD versions, the EZ3 and the G6. It should also be noted that everdrive are working on something for the GBA and though it will probably not be as defining as their efforts for the N64 and older consoles; everything that is not a supercard will play practically everything out of the box and if not then a trip to
http://gbatemp.net/threads/buying-a-gba-flash-cart-in-2013.341203/page-18#post-4756995 will sort the edge cases, said older consoles did not have great flash carts where the GBA did.
You will not find any of those for sale outside of the occasional forum selling section around here.
Best that is still available. EZ4. It has its quirks but if it fits it then it will play it (and if not then
http://gbatemp.net/threads/buying-a-gba-flash-cart-in-2013.341203/page-18#post-4756995 ), supports SHDC memory unlike all the others (give or take what everdrive eventually do). The only thing it does not really do that some others did is real time clock, used mainly for pokemon ruby/sapphire/emerald to have day/night cycles and grow berries -- there are patches but they advance the time when you are playing rather than all the time which still works well enough for pokemon. Most other things the others might have, save for real time save, you can work around.
The EZ 3 in 1 will also do much the same if you want to use it on a DS with a DS flash cart. Where to buy? Afraid I have not really kept up with sites but they are common enough to find on flash cart sites.
