So many good ones have already been said. But I still have a bit of a collection, let's see:
"I don't know how I know, I just know that I know. Ya know?"
-Koizumi
"When you put it that way I can't say I don't get the feeling it doesn't or does it. But you can't say it wouldn't be difficult to assert that claim. Well then. How many negatives did I just string together?"
-Kyon
"You don't die for your friends, you live with your friends. That is how you create a happy future."
-Erza Scarlet
"Don't be distracted by the what ifs, should haves, and if onlys. The one thing you choose yourself, that is the truth for you."
-Kamina
"Anyone can make mistakes. What's important is how you make up for them. All you have to worry about is how not to repeat the same mistake"
-Yusuke Yoshino
"You spout a bunch of crap, but this is crap I can really get behind."
-Viral
"We shouldn't choose our path based on the sins we carry, but we must carry our sins on the path we choose."
-Touko Aozaki