Fairy, psychic and ghost types are three of my favorite types of Pokemon mainly because of their magical abilities and attacks, but grass types are another of my favorites.

Grass type Pokemon have by far some of the best bull**** moves out of any type. Unfortunately, they also have a lot of type weaknesses... Proof to back up what I'm saying and I'll mostly be naming moves that are more on the buff/debuff side with detrimental effects instead of attacks. This will also include any drain moves since they restore the user's HP:

Aromatherapy
Poison Powder
Sleep Powder
Grass Whistle
Spore
Stun Spore
Cotton Spore
Leech Seed
Ingrain (it keeps the user out, but restores their HP overtime)
Synthesis
Absorb
Mega Drain
Giga Drain
Worry Seed
Strength Sap

Those are only detrimental and HP restorative abilities. Aside from these they have a lot of grass_type attacks. Just like any type though only certain Pokemon can use abilities specific to them only.

Hybrid types like poison and grass for example work as well. These garden types have given many trainers a hard time in a battle. You're about to defeat a grass type, but it hits you with Poison Powder and Sleep Powder for both turns then on the third turn uses Grassy Terrain then a powerful grass type move like Solar Beam all while your Pokemon is both poisoned and asleep unless you quickly switch them out. Any player can admit that this has happened to them, even if they aren't grass types that do this to them, if they're avid Pokemon players and have played all of the game.