How do you say you are my best friend in French?

"You're my world and more" is an English equivalent of the French phrase Tu es mon monde et plus.

Specifically, the personal pronoun tu means "(informal singular) you." The verb es means "(informal singular you) are." The masculine possessive adjective mon means "my." The masculine noun mondemeans "world." The conjunction et means "and." The adverb plus means "more."

The pronunciation will be "tyoo ey moh mohnd ey plyooss" in French.