Bulbasaur and Chikorita are the best pokemon in tier S; they will give you the best output in the game. The most prominent pokemon in tier A are Budew and Exeggcute.
Binacle and Bulbasaur (shadow) rank in tier B. The best pokemon in tier C are Chespin, and Hoppip, while Barboach and Houndour (shadow) rank in the D tier.
S Tier

Bulbasaur |
Chikorita |
Chinchou |
Cottonee |
Ducklet |
Tepig |
Vulpix |
A Tier

Budew |
Exeggcute |
Pansear |
Seel |
Growlithe (shadow) |
Slowpoke |
Slugma, |
B Tier

Binacle |
Bulbasaur (shadow) |
Clauncher |
Foongus |
Lotad |
Tentacool |
Wooper |
Oddish (shadow) |
Shroomish |
Turtwig (shadow) |
C Tier

Chespin |
Hoppip |
Lileep |
Litleo |
Litwick |
Poliwag (shadow) |
D Tier

Barboach |
Houndour (shadow) |
Seedot |
Cyndaquil |
Darumaka |
Sewaddle |
Skrelp |
Snivy |
Torchich |
Tympole |