To be blunt, the Grass-type is not great in a vacuum. It's arguably the worst of the three starter Pokemon types -- especially defensively. While it has an average number of resistances at four, it has five weaknesses to be tied for the most along with Rock.
However, the type has a lot of interesting qualities that make it almost necessary for certain teams. There are a lot of special attackers that carry the Grass-type and most of them have access to many unique moves. Many of them are healing-oriented like Giga Drain, Aromatherapy, and Leech Seed.
They also work exceptionally well in the sun, powering up moves like Solar Beam and Solar Blade instantly. If the Grass-type is the only missing link on your team, these ten are likely to be your best bet depending on their specific niches competitively.
10 Meganium (Low-Tier)
What was once a very good wall on both the physical and special ends is now way past its prime. Back in its day, Meganium was known for its bulk, taking hits as it continued to set up dual screens or Leech Seed. It could even fire back with a well-placed Grass-type attack.
Meganium's viability has fallen off a cliff as other Pokemon of its kind, including most other Grass-type starters, would outclass it every passing generation. What has prevented it from getting with the times, aside from its stats, has been its incredibly shallow movepool, which hasn't gotten much else besides the aforementioned screens, Leech Seed, STAB attacking moves, and other Grass-type staples.
9 Bellossom (Low - Mid Tier)
This stone evolution and its counterpart, Vileplume, play very similar roles. Even though Vileplume is much more Special-Attack-oriented and Bellossom's a total jack-of-all-trades, both play the same role of slow, slightly bulky, moderate-HP walls that can Giga Drain and Strength Sap away at their opponent's health. Of course, those differences matter.
Vileplume has a somewhat wider array of Poison-type moves not to mention actually getting STAB for them. Bellossom, on the other hand, has access to Quiver Dance which helps in its agenda to slowly suck the life out of its opponents. Neuter any special attackers with Quiver Dance and deal with the physical attackers with Strength Sap.
8 Lilligant (Low - Mid Tier)
Despite also having access to Quiver Dance, Lilligant is just an average special attacking Grass-type that doesn't have much to help it outrank its peers. Quiver Dance is really the only thing it's got going for it.
As is standard with Grass-type attackers, it's got good STAB options like Petal Dance, Leaf Storm, or Energy Ball, and a few utility moves like Sleep Powder. It also has Chlorophyll as an ability and is on a slightly higher Speed tier compared to other Pokemon with it. This means that it could play the sun sweeper role pretty well. Unfortunately, it doesn't have access to Weather Ball, so that stunts its ability to check Ice-types with Fire.
7 Tsareena (Low - Mid Tier)
The first physical specialist of the bunch, Tsareena's Speed is underwhelming, but its other stats are quite good for the role it plays. It's got better-than-expected defenses at 98 each and, as alluded to, it's got a massive 120 Attack. That works great with its solid movepool.
It can't learn Leaf Blade, but it's got other solid STAB options like Power Whip, Seed Bomb, and its signature move, Trop Kick. It also adds good utility moves like Toxic and Rapid Spin to a surprising number of good Fighting-type moves. Its interesting signature ability is Queenly Majesty which makes it a great wall to priority moves that can stunt certain cleanup sweepers.
6 Leafeon (Low - Mid Tier)
Another physical specialist, Leafeon's 110 Attack and 130 Defense make it stand out in a pool of Grass-types who mainly lean more towards the special side. It's no Kartana, but it can play a similar role if needed. Leafeon is best known for being a Swords Dance wallbreaker and that works best alongside Leaf Blade, Solar Blade if in the sun, and its OK pool of coverage moves.
Speaking of the sun, it's also got Chlorophyll and is faster than Lilligant, so it probably plays the sun-sweeper role better. However, it does possess a physical kink to it rather than a special one (its access to Weather Ball notwithstanding).
5 Sceptile (Low - Mid Tier)
Among the Hoenn starters, Sceptile's unquestionably been the least viable despite having arguably the best stat spread. Granted, it's no small feat to reach the heights Blaziken and Swampert have in terms of competitive usage. When used right, Sceptile can prove to be very underrated.
It's characterized by high Special Attack and Speed, the latter of which can be boosted further upon losing an item thanks to its Unburden ability. Sceptile's best asset is probably its versatility as an attacker as it has tons of options for STAB and coverage moves. It also had a Mega Evolution that, while not good enough to warrant a Mega role in higher competitive tiers, has helped it dominate lower-tier play.
4 Shaymin (Low - Mid Tier)
This only accounts for Shaymin's base forme, as its much more viable Sky forme is a Grass-Flying type. While the latter is a spammable Serene Grace-boosted attacker, the former carries a more all-around bag of tricks with 100 all across the board. That's par for the course with most Mythical Pokemon.
The one common thread between the two formes is Seed Flare, Shaymin's signature move. Though Serene Grace indeed works perfectly with that, base forme Shaymin has to settle for Natural Cure which is still useful in its own right. The moveset to go alongside Seed Flare is the teambuilder's choice, really; it's got a nice pool of coverage and status moves to help fill certain roles on a team.
3 Serperior (Mid- High Tier)
To be honest, this entry could just consist of the several Pokemon with the Contrary ability. That includes another Grass-type eligible on this list: Lurantis. But Serperior has proven to be head and shoulders above them even though it isn't present in Sword and Shield yet.
If you're not in the know, Contrary reverses stat boosts and drops. That means a move like Leaf Storm, the move Serperior is most commonly associated with, would boost the user's Special Attack by two stages instead of dropping it by two stages. It takes a bit to get going since it only has 75 Special Attack, but there should be no stopping it from sweeping the other team after one or two.
2 Tangrowth (Mid- High Tier)
This Generation IV evolution to Tangela is really one-of-a-kind among Grass-type Pokemon as it is the only real tank to come from the type. That makes its access to an array of healing moves like Leech Seed and Giga Drain all the more useful. What's even more useful is its stat spread where it is above average in everything except Special Defense and Speed.
The Speed isn't really a problem and the Special Defense can be fixed with an Assault Vest or by simply healing back the damage with the aforementioned moves. It really helps to have its ability, Regenerator, which makes it virtually unkillable regardless of the approach its opponents take.
1 Rillaboom (Mid - High Tier)
A premier Grass-type attacker in today's game, it's kinda unfair that Rillaboom managed to gain Grassy Surge as a hidden ability and Grassy Glide as an accessible move. That's all it's really known for competitively, and yet that strategy is so formidable that it's imperative to either use it or prepare checks for it.
It's got massive Attack and decent defenses with Speed becoming irrelevant thanks to Grassy Glide's priority on Grassy Terrain. Pro-tip: that is guaranteed thanks to the ability. What's up to the player, then, is the moveset around that. Moves like Knock Off and U-Turn are pretty standard and the Attack can be boosted even further through a Choice Band or set-up from Swords Dance.