There are so many quotes that are special from CLANNAD. Hard to decide lol. Does a quote from a song count though?
If so, mine is from the After Story opening:
"I will remember, no matter the changes. Just this one thing, this one thing, though it's a plain thing. I will show it to you, this one thing covered in glory. Forever and ever, I will hold it dear."
Means a lot to me. Not sure why, it just has that effect =)