Is the Shiny Magby legit.? I have a Mew and Mewtwo. The bad part? You can't really use them in battle until you've gone a little ways, moreso with Mewtwo (Lv.70 I think and thus requiring six badges just to use for one level, then seven badges to keep using it). Mew, meanwhile, could be used after three badges in the same method as Mewtwo (two for Lv.30 battling) as long as it doesn't grow too fast.
So thus I'll suggest Mew (besides, without an Action Replay hack on the GBA Emerald, you can't get it since except via Event). It will be a clone. I could also do Mewtwo, but I highly recommend not trying to use it (yet). Neither, BTW, shows up in your Dex until you have the National Dex, so don't look for their entries until after the E4.