Where is water most commonly found in Universe?

I need to find this out for my class. I can't find the answer in my book and I've looked all over the internet, but no one seems to know for sure. Some say it's Neptune, others say it's Earth, or Europa. At first I thought it was Earth, but there's another part to the question I'm trying to answer asking how this amount of water compares with Earth's. Can anyone give me a definite answer?