I use EZ 3in1

 

...excuse me? I beg to differ.

On my Flash2Advance Ultra cart I have tons of homebrew, including a port of Linux, a fast text editor, a calculator, Minesweeper, 30 varieties of Solitaire, Yahtzee, a very functional text reader, an image viewer, a video player, an audio player, an NSF player, an SPC player, an NES emulator, two SNES emulators, a GB/GBC emulator, a Lumines clone, a paint application, a TI-83 emulator that runs at full speed, a windowing system called GBA WinS, a word jumble solver, two 3D engine demos, and a program to change the real-time clock on my F2AU.

And that's just a fraction of all the games and demos out there. Not to mention all the great commercial games available for the system.

 

That was obviously a pretty ignorant thing to state.

Anyways, I think the best slot 2's are the Supercard and EZ Flash's for homebrew. Actually I've read that Supercards can't be beat for homebrew since most stuff(older .gba homebrew) was coded with those carts in mind. I've got a G6 Lite and have ran into a lot .gba homebrew that won't work with it. I don't know if this is the same with the M3's too. So for homebrew, I think a slot 2 Supercard or EZ Flash will be your best bet.

 

h8uthemost, the Supercard didn't even exist for much of the GBA's lifespan. I'd guess that the most popular GBA flash cart was probably the Flash2Advance, especially with the advent of Pogoshell. For a long time, NOR-based carts were the only ones you can get. NAND-based adapters with RAM for launching binaries like the Supercard and M3 appeared later on. Perhaps you're thinking of early DS programs, as many targeted the CF-based adapters like the GBAMP and Supercard CF.