1) Cacnea/Cacturne: Cacnea and Cacturne are actually my favorite Pokemon in general, not just restricted to the Grass-type. In the era before X & Y, the Pokemon which I felt was most reminiscent of Halloween, my favorite holiday, was Cacnea. That might sound odd considering it's a cactus, but considering its shiny sprite was orange and the design of its face, I grew to like it because it reminded me of a jack-o'-lantern. It's nice that it evolves into what resembles a scarecrow to go along with that little headcannon of mine. Cacturne's stats leave a lot to be desired since it's slow and can't take a hit, but I love the typing and if it does get hits off on the opponent then it's reasonably powerful.
2) Pumpkaboo/Gourgeist: This probably comes as no surprise after seeing my reasoning for liking the Cacnea evolutionary line, but the fact that we actually got jack-o'-lantern Pokemon really made me happy. I would probably like them more than Cacnea and Cacturne if only they had access to Shadow Claw. Lacking Shadow Claw is an immense letdown since I'd love for them to have a strong Ghost-type move that isn't Phantom Force. That said, I love that they come in various sizes which each have their own stat distributions. Like Cacnea and Cacturne before them, they're my 2nd favorite Pokemon among all Pokemon, not just restricted to the Grass-type.
3) Rowlet: I absolutely love birds and Rowlet is my favorite first-form starter Pokemon of them all. Its design is just adorable. I'm disappointed in some ways with how Decidueye turned out since it seems rather removed from Rowlet, but overall I'm glad that we have another line of owl Pokemon. It's tough to say whether I prefer the Rowlet line or the Squirtle line, as my favorite final-form starter is Blastoise.
While those are easy picks for me, Grass-type is one of my favorite types so I have numerous other honorable mentions: Exeggutor, Parasect, Sawsbuck, Serperior, Shiinotic, Trevenant, Venusaur, and Vileplume.
Pumpkaboo/Gourgeist (shiny), Serperior, and spring Sawsbuck (shiny) are the three that immediately came to mind for me so I guess those but honorable mentions go to:
I looked at a list of all the grass Pokemon, and I didn't realize how many grass Pokemon were favourites of mine! It was tough. But I gotta say Roserade, Gogoat, and Rowlet. But Celebi, Decidueye, and Leafeon were all SO CLOSE to making it on my top three.
With Roserade, I enjoy it's androgynous design and love how much it feels like an homage to shoujo manga, really reminds me of Tuxedo Mask.
Gogoat was a super big part of my second Pokemon X playthrough, plus goats are one of my favourite IRL animals.
And Rowlet because I find it comforting. Decidueye definitely has a cooler design, but I had this one day where I was having a really terrible day emotionally and the thought that I'd want to hug a Rowlet randomly popped into my head because they look so soft and cuddly, and literally not even 20 minutes later, I actually found a Rowlet plushie, and I had never seen them in stores before, and it was the only one at the store. Kind of just a little personal story on why Rowlet is an important Pokemon to me.
I'm going by designs I like, and I'll be leaving starters out of this since there's always a grass starter and that would NOT make this a Top 3 xD. So here goes:
I'm so glad to see that so many people have Roserade on their lists!! My favorites are:
Roserade:
I just LOVE this design. I had a Roserade as a part of my Pearl team, which was my first Pok?mon game, and she kicked butt. Roserade also has a decent movepool, imo, especially since she can learn Sludge Bomb (only by TM, I think?)
Lurantis:
I really only like Lurantis in terms of its design. I REALLY wish they could have done more with this 'mon, as I really do think it had a lot of potential. Contary is a pretty good ability, though.
Tsareena:
Grass leggy girl. She isn't too bad when playing through the main game, but I really only used her as a last line of defense. Trop Kick is a nice move, however. She looks amazing design wise! I really seem to like feminine pok?mon ??