Either a bidoof or a starly. I didn't end up using my first 20 bidoof/starly. Yeah, I caught 20 of the same thing. I haven't the faintest idea why.
The first pokemon that i caught and used was Shinx. It was your average male shinx, level three, ability rivalry, liked bitter food, possibly calm nature, caught on route 202 or 203, december 25, 2007 or 2008, forgot which. All of that off the top of my head. My first ever lv. 100, has been traded through three restarts of Pearl (traded to a friend and then traded back to my new game) and then poketransfered to White, traded through one restart and currently residing in my box until I beat the eighth gym leader, at which point he will rejoin my team and help me beat the elite four.