Snivy: This is a grass type as you know, and it can learn some powerful moves such as Leaf Storm and Leaf blade. Snivy also happens to be strong against water, rock, and ground types. The cons about snivy is that later on towards the end of the game, snivy will be in big trouble because as you know, grass is weak to ice. Since you have to battle a really strong ice type, remember to bring a lot of reviver seeds if you want to proceed with snivy.
Tepig: Tepig is another one of the Unova starters that you're able to choose in this game, and it also learns some great moves including Rollout, flame charge, and heat crash. The only bad thing is, Tepig is weak to rock, water, and ground. That might put Tepig in for some trouble when it comes to a certain point in the story when you go and rescue a certain pokemon.
Oshawott: The last of the Unova starters, and possibly the best, you have Oshawott. Oshawott will learn some moves like aqua jet, razor shell, and a few other good ones too. Plus, its strong against rock, ground, and fire. Some cons about Oshawott, would have to be its weaknesses to electric, and grass. Otherwise, Oshawott would make a fine choice for the hero or partner.
Pikachu: Pikachu is well known for everyone, and it can learn a bunch of good moves, such as electro ball, thunderbolt, and quick attack. Its great against water and flying types too. And one great part about Pikachu is that its only weak to one type, and that's ground. That makes Pikachu one of the best choices for the hero or partner
Axew: Finally, there's Axew. Axew can learn a bunch of really strong moves like Dragon Claw, Dual chop, and Assurance, as well as Dragon Dance, which raises your attack and speed. Axew is especially effective against other dragon types, and its weaknesses are dragon and ice, but if you train a lot, Axew can beat up just about any pokemon. Axew's strength against other dragon types also makes it easier for you to finish the game.
So to wrap this up, each pokemon listed here is a good choice, but I have some recommendations too. If you're just beginning to play, I suggest you chose Axew as your hero and Tepig as your partner. That way, you can complete the game without any trouble. If you're more experienced with this game and its story, then I suggest you put together your own team and experiment with the different combinations available. Otherwise, start playing and good luck!