If you are the copyright holder over the videos that you created then you can use the standard phrase:
"Copyright @ [name & year]. Any illegal reproduction of this content in any form will result in immediate action against the person concerned."
If you're using the content you don't own instead, in my opinion, the best description you can put under your YouTube videos should be the following:
"This channel and I do not claim any right over any of the graphics, images, songs used in this video. All rights reserved to the respective copyright owners."

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