Grass tends to be one of those types I have loads of fave Pokemon, so yeah I'll list what I call the "best of the best" in terms of Grass Pokemon!
I love Whimsicott. Not only is it adorable as heck, it's also an amazing abuser of status effects thanks to the Prankster ability. Oh, and it's now part-Fairy meaning she can stop Dragon-types cold in her tracks.
Always been one of my go-to Grass-types. It's a powerful special attacker and I was so excited to see Roselia got its own evolution in 4th Gen, they did a good job making it a usable one.
Sawsbuck (and its pre-evo Deerling) have what I think is one of the most clever form concepts ever, a form that changes with the seasons. It's also got the stats to hit hard as well, plus it has a pretty unique type combination.
Who doesn't love the royal majestic snake? Serperior and its evolution line are not only pretty decent at defensive roles, they have the speed to set up quickly. Its movepool may lack in variety but makes up for it in its beauty.
Shaymin is one of my fave Grass legendaries design-wise. It goes from being a cutie in Land Form to a badass in Sky Form. I also love how it's all tied with the Gracidea flower.
I've never used Chesnaught yet, but its evolution line really nails the whole "cute to badass" thing. I was sad at how many hated Quilladin so I was hoping the final evo would fix that issue and yeah I'm happy with what we got. It's also a pretty decent physical attacker with the bulk to take hits.
She's the notable abuser of confusion-free Petal Dance thanks to her Own Tempo ability and while she learns nothing but Grass and Normal moves, she can be a very feisty special attacker with set-ups from Quiver Dance.