I usually start with a Mudkip or Torchic. For some reason, I just don't like Treeko. Never have liked Grass types much.
These Pokemon are good when you're starting off...
- Poochyena
- Shroomish
- Ralts (they are hard to train at first, but definately worth it)
- Zubat (I used to hate Zubat, but then I decided to train it as a once-off and fell in love. Crobats PWN!)
- Taillow
- Beautifly/Dustox (surprisingly, these two can be okay in the lower gyms. I've always dropped them, though)
- Marill (if you don't start with a Mudkip, Marill is a good idea)