Note: I won’t be including Island Scan or Ultra Wormhole Pokemon, as they are harder to get than the ones in the main Pokedex.
Decidueye has solid Attack and gets nice moves like Leaf Blade and Spirit Shackle. It is a starter though, and it is frail, meaning you’re out of luck if you didn’t pick it.
With Quiver Dance, Sleep Powder, Petal Dance, and Own Tempo, Lilligant can be truly fearsome under the right circumstances. However, it’s still quite frail, and it has a pitifully shallow movepool.
Great Attack and solid Defense make it a hard-hitting, fairly bulky attacker. However, the other defensive stats it has are bad, and the Mossy Rock can be an annoyance to get to. It has a shallow movepool, but the moves it does get are good.
If you get one with Contrary and Leaf Storm, it murders. However, that is its Hidden Ability. It’s very slow, but it does sport a good Attack and pretty nice defenses. It has a surprisingly decent movepool, with access to Leech Life, Brick Break, Leaf Blade, and Superpower.
Terribly slow, and not very physically bulky. Morelull does get access to the amazing Spore though, and Shiinotic gets nice moves like Giga Drain, Dazzling Gleam, and Sludge Bomb.
Not very good. Devastating weaknesses to Fire and Flying types, plus Ice, Poison, Bug and Rock. Awfully slow, and the only halfway decent stat it has is a base 95 Attack.
You can get a Bounsweet from a trade on Akkala, meaning it gets boosted experience. It’s very common, and has a phenomenal Attack, though it is slow. Queenly Majesty is pretty good, and it has moves like Trop Kick, U-turn, and Zen Headbutt.
Too slow and too defensive to be worth much in-game. It does have a few decent moves, but that isn’t enough to make it that good. It also comes at a very low level and evolves at a very high level.
Being a trade evolution weighs it down, but it does have a nice movepool and solid Attack. It’s frail and quite slow, however (sensing a pattern?).
Post-game only, therefore it’s not good. I would go into more detail, but there’s no point.
You can only get it on Poni Island, and it’s an incredibly rare find from Seafolk Village. Steelworker is a cool Ability though, and it has great Attack and respectable defenses. With moves like Anchor Shot and Power Whip, it’s quite good if you don’t mind the wait.
Too defensive for in-game. It’s too weak for battles, with numerous weaknesses and not very many good moves. It has crippling weaknesses, it’s slow, and doesn’t even have a BST of 500.
Slow, but a great special attacker. It’s decently bulky as well, and has good moves like Psychic, Flamethrower, and Energy Ball. That weakness to Ice really harms it, however.
Legendary, meaning not all people would want to use it. It has a terrible weakness to Poison, and it’s really annoying to find. It does get good moves like Superpower, Megahorn. and Wood Hammer.
Based on this, we can narrow the field down to a few Pokemon.
Lilligant should be your go-to special Grass type, but do keep in mind that Grass doesn’t have a whole lot. If you don’t mind waiting until almost the end of the game, Alolan Exeggutor is better, but it just takes so long. Decidueye is a great Grass type if you pick it as your starter, but only then, as you can’t get it otherwise. Leafeon and Tsareena are great physical attackers as well, and Tsareena ever so slightly edges out Leafeon due to a higher Attack and more varied movepool.
Hope I helped!