Choose whether to use a colon or a semicolon in this sentence!
i'm not sure any lunch can beat we had the best pizza in the whole world.

This is a tricky question, especially because we are not offered a context for the sentences. But, in my interpretation, the best option would be a colon.

A semicolon connects independent sentences - it has other uses, this is just one of them. That means that, in this case, both sentences should present a complete meaning in themselves. One should not need the other to make sense. However, the first sentence, on its own, is a bit lacking, especially because we do not have an object completing the verb "beat" at the end.

A colon introduces a sentence that is explaining, adding meaning, to the one before it. That seems to be what happens in this case. Why would no lunch beat? Because we had the best pizza in the whole world. The meaning of the first sentence is now complete.