Now we can get into all of our current choices and explanations.
1-Drops
Elvish Mystic- Our most basic and standard ramp, essential for quickly assembling our army.
Hardened Scales- One of my favorite cards in standard right now. Abosutely wacky.
Servant of the Scale- Benefits greatly from Hardened Scales. Can enter the battlefield with an extra counter, and leave the battlefield boosting another creature with his own power plus 1.
Other options you could include- Charging Badger if you pack pump like Aspect of Hydra. Honored Hierarch if you like renown and extra ramp.
2-Drops
Avatar of the Resolute- Excellent card that contains two of the keywords that we love to see, reach and trample. He's great in the early game and in the late game.
Den Protector- Extremely versatile card that can help us keep the game rolling later on, and can't be chumped very easily.
Sylvan Caryatid- Great ramp card with a great rear end, and not too easy to kill.
Evolutionary Leap- One of my favorite new cards from Origins. Allows us to stay in the game if we're facing a deck loaded with removal.
Ainok Guide- Without doubt not the greatest 2-drop but great to get a 3/3 for 2 with Hardened Scales.
Other choices might include Solidarity of Heroes/Setessan Tactics/Heir of the Wilds. Just to mention a few.
3-Drops
Managorger Hydra- Escalates very quickly if not dealt with, and many overlook the fact that he has trample. Ideally he is a T-2 play.
Reverent Hunter- Would be infinitely better if he had trample, but he often poses a large threat that must be blocked.
Nissa, Vastwood Seer
- She is a really cool planeswalker that definitely has a spot in any midrange builds.
Deathmist Raptor- Great recurring creature that pairs very well with Den Protector.
Courser of Kruphix- Great for sifting through your deck for the things you really want, and the lifegain is impeccable.
Bow of Nylea- Excellent versatility. Pairs well with Hardened Scales, can keep you alive, and deathtouch/trample pair so very well together.
Inspiring Call- Indestructible and card draw that pairs very well with the +1/+1 counter theme.
Other options may include Shaman of Forgotten Ways/Hornet Nest/Boon Satyr
4-Drops
Surrak, the Hunt Caller- My 4-drop of choice. Not only does he have haste in most cases, but he gives it as well. Pushing through for 5 extra unexpected damage will win you games.
Collected Company- A really broken card that provides green with creatures at instant speed. The only downside is you must build your deck around it.
Nylea, God of the Hunt- Indestructible, provides trample to your other creatures, and can provide pump. What more could a green-lover ask for?
Other options may include Polukranos, World Eater/Frontier Siege/Temur Sabertooth/Nylea's Disciple
5-And-Up
Whisperwood Elemental- Gives great board presence, and can save you from total board wipes. Excellent in mainboard or sideboard against control.
Nissa, Worldwaker- Great if you play a ramp-oriented build.
Woodland Bellower- Definitely a great card that can dig out your best creatures.
Gaea's Revenge- Incredibly powerful if your curve allows it.
Hornet Queen- Can save your life and take theirs. One of the best and only flying sources in mono-green standard.
Other options could be Terra Stomper/Outland Colossus/Heroes' Bane
X-Drops
Hangarback Walker- Pairs amazingly with Hardened Scales and just as well with Evolutionary Leap. A must have in this build.
Mistcutter Hydra- Just because he has pro-blue does not make him solely a sideboard card. Haste is an invaluable asset.
Animist's Awakening- If you like ramp.
Genesis Hydra- If you love digging for answers.
Keep in mind that these are not the only valuable cards of the color green that are in standard currently, they are just some of the most notable in my own opinion.