My Favorite was actually the one taugt by Conan to Genta and Mitsuhiko.
Once those words leave your mouths, you will never be able to take them back.
Words are like swords.If you use them the wrong way, they'll turn into ugly weapons.There are cases of broken friendship that could have lasted a lifetime if not for verbal abuse.Once you severe your ties, you might not see each other again.
~Edogawa Conan
(Detective Conan movie 15: Quantum of Solace)