Grass types are tied with Rock in having the most weaknesses out of all types with five.
Since Generation I, a particular asset of Grass types is being the only type that is immune to Leech Seed. As of Generation VI, Grass-type Pokémon are also immune to powder and spore moves, such as Sleep Powder and Stun Spore. Additionally, Grass Pokémon are the only ones affected by Rototiller and Flower Shield. Those moves raise both attack stats or the physical Defense stat, respectively, when used.
Forest's Curse adds the Grass type to the target's types.
Grass-type attacks are resisted by seven types, so they're tied with Bug as the most resisted type. Those resistant types are best covered by Rock and Ground. As of Generation VI, Dark and Ghost have neutral matchups against every type that resists Grass.
When Grassy Terrain is in the effect, the power of Grass-type moves is increased by 30% (50% prior to Generation VIII) if the user is on the ground.
Disclamer: Rarity of an individual Pokemon can and does vary depending on the spawn location. In the table below the lowest rarity is presented. Please check individual routes for more information on the rarity of your desired Pokemon.
Disclamer 2: All Pokemon names and locations contain a hyperlink to their pages. Links have been colored black for design purposes. Please note that not all pages are of quality or even exist, work in progress.