I'd say M3 Lite, it's really good. It's quite expensive though.
Another good choice might be the EZ-Flash III. Many people say it's the best flashcart.
If you also have a DS the EZ-Flash 3in1 Expansion Pack might be interesting: it's really cheap and you can flash a GBA rom to it using a Slot-1 cart.
Hmm...well I do have an M3 Real which I currently use on my DS Lite but I've always wanted to play GBA games on a real GBA. I'm not really for playing the games on my DS. It just doesn't feel right.
So, I could use the EZ-Flash 3-in-1 and flash 'one' GBA game to the cart and then play it in a GBA?
M3 Perfect Lite MicroSD whipes the floor, walls AND ceiling with all other GBA comp cards.... hands down. And yes, you can put it in a real GBA SP (GBA and Micro too!)