〜に限る
(X) is the best, nothing beats
Noun / Verb dictionary form / ない-form + に限る
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に限る means "the best thing is": 疲れた時は、寝るに限る, "when you're tired, the best thing is to sleep". 限る means "to limit", so it's "limited to this one": nothing else will do.
It's the speaker's personal conviction, often about everyday pleasures: 夏は冷たいビールに限る, "in summer, nothing beats a cold beer"; 寒い日は温泉に限る.
It follows a noun or a verb in the dictionary or ない-form. The first half often sets a situation with は or 時は.
Compare に越したことはない, which is more general advice, and に限って, "of all times", which is completely different.
Examples
疲れた時は、寝るに限る。
When you're tired, the best thing is to sleep.
夏は冷たいビールに限る。
In summer, nothing beats a cold beer.
旅行は春に限る。
Spring is the best time to travel.
嫌なことがあった日は、早く寝るに限る。
On a bad day, the best thing is an early night.
寒い日は温泉に限る。
On a cold day, nothing beats a hot spring.