The original EZ3 (
http://s186.photobucket.com/user/trikon000/media/gbatemp/ez3.jpg.html ), quite possibly depending upon how you feel about needing windows XP and not being able to chuck a new ?SD in when something goes wrong. If you can find one today at a reasonable price then you are doing very well. Otherwise they are much like the EZ4 but have real time clock, a functioning cheat engine and savestates (not great but not worth dismissing either). Indeed if you can get a working one for cheap from a friend/someone that does not know what they have then you could probably resell it and finance another flash kit.
The EZflash 3 in 1 (
http://wiki.gbatemp.net/w/images/thumb/0/08/3in1-1.jpg/350px-3in1-1.jpg ). This is an expansion pack aimed at use with a DS slot flash cart on the DS, you will probably only be able to find the DS lite version today though a simple case mod to allow it to fit means you can use it on a DS as well. Technically it can work with a GBA (just write something to NOR) but it is very annoying if you want to swap between games. Oh I suppose it has rumble as well, not that much uses it. If you have a DS slot cart though then it will allow you to play just as many games -- everything that is not a supercard or team cyclops clone of the same will play basically everything.