It depends on your preference and what other pokemon you've planned for your team. The first gym leader you face is based off which starter you pick anyway, being strong against yours.
The Pokemon You Can Choose:
Snivy is a grass type, its final evolved form is Serperior and it's speed state is the highest.
Tepig is a fire type, its final evolved form is Emboar (it becomes fire-fighting when it evolves), and it's attack state is the highest.
Oshawott is a water type, its final evolved form is Samurott, and it's special attack is the highest state.
(Jalorda can also gain another ability through the Dreamword called Contrary and All moves or abilities that would normally increase a stat will decrease it, and ones that would normally decrease a stat will increase it. This also applies to self-inflicted moves, such as Charge Beam, Close Combat and Leaf Storm. This does not include Baton Passed stats. Credit to Bulbpedia)
The Unova Starters
Snivy
Oshawott
Tepig
Snivy
Base Stats:
HP: 45
Attack: 45
Defense 55
Sp. Attack: 45
Sp. Defense 55
Speed 63
Base Stats at Final Evo:
HP: 75
Attack: 75
Defense 95
Sp. Attack: 75
Sp. Defense 95
Speed 113
Snivy is a grass type, its final evolved form is Serperior.
Choosing Snivy might make the game a bit more tough, but hey, who doesn't like a challenge? 
Oshawott
Base Stats:
HP: 55
Attack: 55
Defense 45
Sp. Attack: 63
Sp. Defense 45
Speed 45
Base Stats at Final Evo:
HP: 95
Attack: 100
Defense 85
Sp. Attack: 108
Sp. Defense 70
Speed 70
Oshawott is a water type, its final evolved form is Samurott, it's special attack is the highest of the three.
He is more of an all-rounder.
Tepig
Base Stats:
HP: 65
Attack: 63
Defense 45
Sp. Attack: 45
Sp. Defense 45
Speed 45
Base Stats at Final Evo:
HP: 110
Attack: 123
Defense 65
Sp. Attack: 100
Sp. Defense 65
Speed 65
Tepig is a fire type, its final evolved form is Emboar (it becomes fire-fighting when it evolves),
It's attack state is the highest, but it is also the slowest of the three.
Well..
Who are you gonna choose?
I did not make any of these pictures, just googled for them,
if you'd like to add anything please do, all contributions are welcome!
