I like a lot of Grass types but I only really like a few. Most of my favorites are based off of some of my favorite plants/fungi.
Amoonguss is one of my favorite Gen. 5 Pokémon. The clever Poké Ball pattern and fungus inspiration are a great combo. Lol at the weird circle mouth. I also like the family's name: Foongus Amoonguss.
Cacturne is also one of my favorite Grass types. One of the best designed Grass types and also has some interesting biology and habits. Their blood has evolved into sand and they hunt weary travelers in the desert at night.
Another one of my favorite Grass type Pokémon is Tropius. It's a very unique Pokémon with the sauropod/banana plant combo. The leaf wings are clever and it's shiny pallette is really nice. I also like the edible fruit growing on its neck.
Last but not least, Gogoat. I gotta give the new Generation some love this week. Gogoat has been really helpful with weakening wild Pokémon so I could catch them. I haven't restored the power in Lumiose City yet, but I imagine that riding them is a fun experience.