Okay, okay, there may still be proof pending that this is ACTUALLY the most important interval but we think it's pretty amazing. The interval in question is the perfect 5th. A perfect 5th is built up of the 1st and 5th intervals of a chord. You could also consider this an interval made up of 7 half steps.
Why this interval is important:
This interval acts as a framing interval for more complicated chords. If you play an open 5th in your left hand, you can fill out the rest of the chord in your right hand. To put this into perspective, think of a jigsaw puzzle. The perfect 5th is like the corner pieces that frames the picture. The major or minor 3rd intervals are what fill in the picture and give us a sense of what kind of picture we are working with. Major third will be a happy picture... Maybe kittens or a beautiful landscape. A minor third will evoke more spooky darker sounds like a haunted house or stormy night.
A great way to begin to experiment with this is to play a perfect 5th very low on the piano with your left hand and just let it ring. Then, with your right hand begin to explore two different 5 finger scales in your right hand. Begin with the major 5 tone scale and then play the minor 5 tone scale. Notice how the perfect 5th sounds gentle and happy when it is colored with the sound of a major scale. When you are using the minor scale the perfect 5th suddenly sounds and feels moody and ominous.
So that is why the perfect 5th is the most important interval of all time. It contextualizes the rest of the chords you are about to play and serves as the root that all other chords are built off of.
Have fun building chords!