My first was TRE. I found an online GB emulator that had a list of ROMs you could choose from to play, one of them was TRE, and being the Team Rocket nut I am...
Yep, I joined PokéCommunity because of hacking. I'd wanted to make a Team Rocket game for a long time, discovered the Showcase forum when I was googling for more info about TRE to see how it had been made, and signed up.
I shall be unique by saying that not only was Shiny Gold NOT the first hack I played, I haven't even played it yet. I have my seven-year-old Crystal ROM that's still working fine, and I don't feel the need to upgrade from that for a few extra subplots and fancier graphics. There's more nostalgia attached to my old ROM. :P