Strictly competitively speaking it goes as following:
1. Shaymin-S: Ridiculous speed, Serene Grace backed by STAB Air Slash and Seed Flare. It has an 80% chance to double the damage it does by usiong STAB Seed Flare and then can flinch a pokemon to death with Air Slash.
Attainability: Hard.
2. Celebi: Amazing defensive spread and better abilities than Shaymin (not Shaymin-S). It can act as either an Offensive, Defensive, or Baton Pass threat and will do its job almost without fail. Poor defensive typing (Psychic/Grass is terrible), but the resistances it has are all commonly used types.
Attainability: Very Hard.
3. Breloom: Breloom is nearly unbeatable if used correctly. Utilizing the moveset of Substitute, Spore, Leech Seed, Focus Punch with Poison Heal it can outstall almost everything. Come in on something you know to be slower, and you will be in for quite a while. Alternatively, you can abuse priority with Breloom by using Spore, Swords Dance, and Mach Punch. The best Grass physical attacker.
Attainability: Very Easy
4. Roserade: Roserade is arguably better than Breloom with its great speed, incredible power, great STABs and a decent movepool. It can act as either a lead or offensive threat, functioning as either the best Stall lead or an amazingly powerful Special Attacker. Outside of Shaymin-S (who is hard to attain) Roserade is the best Special Attacker of the Grass types.
Attainability: Very Easy.
5. Shaymin: Shaymin puts itself at fourth place. It is good but is largely outclassed by Celebi, as Celebi has a similar but better movepool. The lack of a Psychic typing is good in that it isn't absolutely WTF pwned by Bug types, but the resistance to Fighting that Celebi has is very useful defensively.
Attainability: So-so
6. Venusaur: Right now, Venusaur has a lot of hype, but that is strictly in the UU Tier. Sleep Powder + Swords Dance is nice, but is more effectively used by Breloom who has better Attack, better STABs, and a more reliable Sleep move. Basically use any of the above Grasses over Venusaur.
Attainability: Depends on the version.
Trust me when I say that no other Grass type is worth using in-game. People have recommended Torterra, Sceptile, Leafeon, Tangrowth, and others, but the ones I talked about here outclass them 100%.
Hope this helps.