For me, it's Hecarim.
"Behold the might of the Shadow Isles".
Hearing that just gives me the feels like ahhhh. So epic. I love it.
If you don't quite know what quotes give you the chills, watch this video. (It's the latest compilation I could find with all the voices and quotes from each champion. Sorry that they're impersonations) http://www.youtube.com/watch?v=xVcPu9hgYSc
Should help find yourself.
Quote:"They called me a heretic... now they are dead."
"The moon will rise... the night will last forever."
Basically all of Diana's quotes are amazing.
...well, I like to think.. stuff.
Also, when Leona says "I will protect you.", I truly believe her =)