Best.
Three way fight between various types of EZ3, M3 and either the G6 or a 1024Mbit EZ2.
Chances of finding one nowadays are not good at all.
Right now GBA flash carts amount to
EZ4
Fire cards
Supercards.
The EZ4, especially if you can get the lite deluxe version and do not mind case modding it, is about the only real option. Fire cards top out at 256Mbit or 32 megabytes but you will be lucky to find one than 16 megabytes in size and supercards are generally bad, if you are offered one as "just pay for shipping" or "buy us a beer and..." then sure but otherwise I would not. Supercard compatibility is not great (every other flash cart is usually* apply a simple patch and if it fits it plays), I hover between 70 and 80% with slowdowns in several and that 70-80% does miss out several things, this is also why despite them not being made in several years you can still find them.
*
http://gbatemp.net/threads/buying-a-gba-flash-cart-in-2013.341203/page-18#post-4756995 for every game that troubles most things that are not a supercard, also good for the EZ4 which is what your only real option is. This does include some potential issues for pokemon as there is no RTC support (RTC was not used on many games so it was dropped for the EZ4), there are patches that do quite well though (growing berries is the main issue but there are cheats for a reason).
Back on the EZ4 though.
There are 4 models though you will tend to only have a pick of two.
1) The original EZ4.
2) The EZ4 lite deluxe.
If by some chance you find a EZ4 lite compact, M3 professional or supercard rumble then do not buy them as their GBA capabilities are limited or nonexistent in the case of the rumble.
The original EZ4 is GBA cart sized, uses miniSD (and you do want a miniSD rather than an adapter) and generally works.
The EZ4 lite deluxe is DS lite dust cover shaped (though you can mod the case a bit and have it fit in a normal sized GBA slot), uses microSD (up to 2 gigs) and has a bit more memory so you can play any game from PSRAM and it also has a bit more NOR. Just to round it up the EZ4 lite was almost DS lite GBA slot sized, used microSD and was otherwise the same as the original EZ4.
On PSRAM and NOR.
GBA games require fast memory to run, supercards do not have memory that is up to snuff for all games hence the issues with them and similarly NAND memory (which is the stuff that SD cards, USB drives and the like use) is too slow so things get catapulted onto fast memory to be run on the GBA (it is why the carts are more expensive than the likes of the DS, also why emulation of GBA games from the DS slot is troublesome).
The original EZ4 has 16 megabytes of PSRAM (aka 128Mbit) and 32 megabytes (aka 256Mbit) of NOR.
The EZ4 lite deluxe has 32 megabytes of PSRAM and 48 megabytes of NOR.
The largest GBA games are 32 megabytes, there are not many of those (I have a list on the link) and most things top out at 16 megabytes so the original EZ4 can fit any dumped game and likely any hacked games in the future. Said games being so small also means the lack of SDHC (if it is above 2 gigs consider it SDHC) is not quite so troublesome as it might be if you faced the same restriction for the DS.
The EZ4 was probably discontinued after being brought back for a final run in 2009 I think it was. They stuck around for a while but for the better part of a year we saw wither a run on them or at least a lot of interest, a subsequent lack of stock in many places and then a bit of price gouging. Someone then found a crate of them or the EZTeam were coaxed back into making so a few shops have them again (
http://www.realhotstuff.hk/EZ-Flash-IV-_p_76.html and
http://www.realhotstuff.com/EZ-Flash-iV_p_76.html being an example, before you ask .hk is Hong Kong where .com is somewhere in the US and tends to ship a bit faster).
The EZ4 lite deluxe is a bit harder to find.
http://shop.01media.com/brand.asp?ID=434 is about the only place I know of that still does them.
How this will change as time goes on... who knows. Nobody has yet stepped up and said they are making a proper GBA flash cart as has happened on many other systems though and I would not feel confident putting money down on it happening any time too soon.
Alternatives
There is the k101 which is a proper clone of the GBA (less GB/GBC which by the way you will need to emulate on basically every GBA flash cart, that is OK though as goomba and goomba color are great). As with most clones it did not exceed the original and there are a few issues here and there but it got very close.
If you have a DS then you can buy an expansion pack (the EZ 3 in 1 being the best option for most). Here you run a program on your DS slot flash cart and manage it from there. Compatibility wise it is the same as the original EZ4.