I will run you through all 3.
Snivy
This guy is probably the hardest in the beginning because he is weak to the first 2 gyms. He also is not much use it the gyms (except the first) until the 5th( Clay, Ground Type user). But after that he is kinda useless again until the 8th Gym(Water). This guy is the most promising though because of his decent defenses and his 113 Base Speed as Serperior! He also has a decent movepool including the Dual Screens and SubSeed.
Tepig
This guy will be easy to level up and will be able to destroy a lot of stuff along the way. He is only weak to the 5th, 7th, and 8th gyms. But by then you will have lots of other Pokemon. He has a great movepool including Flare Blitz, Head Smash, and Wid Charge! While Emboar may be low on speed, his attack is usually enough to keep him up.
Oshawatt
This guy is super easy to use against the gyms being weak to only the 4th(Electric). Very rounded stats and great moves. He has a decent movepool as well but is not as helpful late-game as the other two starters.
That is all I can say based on their stats, moves, and the competition you will have to face! I hope you choose the best for you!
