There's Only One Truth! (真実はいつも一つ!, Shinjitsu wa Itsumo Hitotsu!; English TV "One Truth Prevails") is the catchphrase of Shinichi/Conan mostly used in opening themes of the series. He mostly says it when he is talking about the truth about what happened in a case or evenly is talking in a special and starts to point and telling his catchphrase.
It was officially never been placed in the manga, but it later did. Shinichi's catchphrase is probably a pun on his first name which can be translated as "One Truth", but the kanji is different which is actually mean "New One".
In the United States, Shinichi's catchphrase has been changed into "One Truth Prevails" under the release of Case Closed. That catchphrase is more used than the original catchphrase in the dubbed version then in the original version.