Sceptile!!!!!
Normally, I pick the fire starter. I picked Torchic on my first go in Emerald, and eventually got a Blaziken. I loved that it was fighting/fire, but in the back of my mind I kept thinking about Treecko, so I restarted my game and chose it, nicknaming it Gex (after Gex 64).
This was the first grass type I'd ever properly used on my team, and as a Sceptile it pulled me out of a lot of tough scrapes. I am now playing through Emerald again with Sceptile, and not looking back! :D
So Sceptile, cause it's bad-ass and very loyal! (To me!) :D