Starting from least to greatest, here we go:
#5 - Mathematics
Where would we be without mathematics? Probably an unindustrialized society. I love it, which means living without it would make me want to drive a spear through my heart. :)
#4 - Books
Without books, I would cease to learn. And without learning, I would drive myself crazy. And in driving myself crazy, I would cease to live.
#3 - Water
This is obvious, but very true. Drinking water is my life vitality. End of story.
#2 - My Mac
I would seriously become destitute without it. You have the internet, I-tunes, written school notes, written school papers, etc. all at your fingertips. Very useful machine.
#1 - Jesus Christ
Without the Life itself, we are in bad shape. The Life (Jesus, John 14:6) came to give us life. I could not live if it was not for Him.
Sola Deo Gloria!!!!