If you plan on keeping your starter, pick Tepig. It's *slightly* better than the others thanks to its second typing and more diverse movepool.Jellicent, Amoonguss, Excadrill, and Galvantula are all fun, as is Braviary if you can stomach the grind to evolve Rufflet.
If you plan on ditching your starter, pick Oshawatt. You'll get the fire monkey, you and you an catch a Blitzle and replace Oshawatt with Tympole for a decent core before the second gym.
No matter what you pick, I'd recommend filling your party with Lillipups and replacing them as you catch more 'mons. Pickup is great and if you're lucky you might never have to buy an item until at least halfway through the game, if not more.
