Presenting a comprehensive list of the best monogreen vanilla creatures per CMC, by highest power and toughness, to date (only "true" vanilla creatures, and not "virtual vanillas" like WIld Nacatl):

1 Mana:
Green Norwood Ranger, 1/2
Green Trained Jackal, 1/2
Green Woodland Druid, 1/2

2 Mana:
GreenGreen Kalonian Tusker, 3/2
GreenGreen Swordwise Centaur, 3/2
GreenGreen Elvish Warrior, 2/3

3 Mana:
GreenGreenGreen Leatherback Baloth, 4/5

4 Mana:
2GreenGreen Rumbling Baloth, 4/4

5 Mana:
4Green Panther Warriors, 6/3
3GreenGreen Silverback Ape, 5/5
3GreenGreen Hollowhenge Beast, 5/5
4Green Pheres-Band Centaurs, 3/7

6 Mana:
4GreenGreen Vorstclaw, 7/7

7 Mana:
4GreenGreenGreen ... (see all)