Me? Well, the first ROM Hack (That I knew was one) I ever played was something called Mario VIP. If you don't know what that is, look it up.
First Pokemon ROM Hack? Shiny Gold. It got me to join the site, even though I was lurking around as a guest for about... half a year. I was eventually going to join anyway.
EDIT: Actually, I just realised. The first ROM Hack I ever played is Ketai Denjuu Telefang (Or better known as 'Pokemon Diamond'). I know that's an actual game, but it was the translation, and it was renamed, and I played it as a ROM. Technically it's a ROM Hack. I played it a long time ago, and I mean it was before the third gen, I believe. I might be remembering it wrong, though. (And this also is the first hack I played. But I'm keeping VIP up there, since it was the first one I knew was a ROM Hack.