I'll go off the assumption you mean in Reborn. Let's look at the details.  Bulbasaur: Good typing, gains great support later on and can both take and dish out modest damage relatively well. Great if you don't plan on using Venusaur as your primary damage-dealer. 
Chikorita: Use it if you like a challenge. It is very difficult to effectively use. 
Obtainable later in-game. 
Treecko: Best starter It's not too good early on, but if you give it attention and the right training, it can deal medium-high damage. Thing is, in Reborn, that's tough to pull off. 
Turtwig: Excellent bulk and unique typing make it great for Ground-type offensive strategies. It can take hits quite well and functions as a good self-sustaining party member once it's fully evolved. 
Obtainable later in-game. 
Snivy: Tough to use at first, and relatively weak unless it has Leaf Storm and Contrary. If you can get both of those, then it's a wrecking ball limited only by its poor level up moveset. An excellent offensive choice if you can give it the right moves. 
Quilladin: Tough to use at first, but once it evolves and gets a good movepool, it's an efficient enough wall, capable of dealing modest damage. Bulletproof is the ability you'll want to go with. 
Obtainable later in-game.  
Your best choice is either Bulbasaur or Turtwig, in my opinion. Bulbasaur for Special Attacks, Turtwig for Physical.