Hmm well dual types are better overall so lets list the starters first.
1. Bulbasaur
2. Chikorita
3. Treecko
4. Turtwig
5. Snivy
6. Chespin
In my opinion
1. Turtwig
2. Bulbasaur
3. Chespin (Can be obtained later in the game)
4. Snivy
5. Treecko
6. Chikorita (Far behind the rest, can be obtained later in the game)
Also keep in mind to use moves that benefit grass types as a whole. You can get many pokemon with Chlorophyll or Leaf Guard etc. It may be better to skip on Bulbasaur since Grass poison you can get quite easily after the first gym, or if you can find a Budew an evolve it into Roselia through happiness before the first gym. Hoppip/Skiploom/Jumpluff can be obtained around the same time on a windy day. After a couple of badges, You can catch yourself a Nuzleaf and train it into a powerful Shiftry with a Leaf Stone of course. I don't know where Exeggcute, Shroomish, or Lotad can be found, but they'd all also be good on your team.