〜に越したことはない
it's best to, you can't go wrong with
Plain form / Noun + に越したことはない
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に越したことはない means nothing could be better than this: 用心するに越したことはない, "it's best to be careful". 越す means "to go beyond", so it's "nothing goes beyond this".
It's used for common-sense advice where the point is obvious: cheaper is better (安いに越したことはない), earlier is better (早いに越したことはない), it's good to have savings (貯金はあるに越したことはない).
It often implies "but it's not essential": the ideal would be this, even if it's not always possible.
Compare ほうがいい, which is direct advice. に越したことはない sounds wiser and more general. Nouns and な-adjectives can take である, but often attach directly.
Examples
用心するに越したことはない。
It's best to be careful.
安いに越したことはない。
Cheaper is always better.
準備は早いに越したことはない。
When it comes to preparation, the earlier the better.
貯金はあるに越したことはない。
It's always better to have savings.
事故が起きないように、気をつけるに越したことはない。
It's best to be careful so that accidents don't happen.