Yeast, because without yeast we could not create mass quantities of bread for the masses and we couldn't make wine or cheese or other resources that are used in so many industries, it's also very useful in genetic modification due to it's simple nature you can modify it and add and edit Gene's like makin fb it glow in the dark or bioluminescent. Lastly, we can use its process to make biofuels for space travel and for a symbiotic relationship with other plants and animals.
