According to the rules, you should use "better" when comparing two things and "best" when comparing more than two, so the second sentence is correct. In everyday English, though, people often use "best" even when there are only two things (and similarly with other superlatives), and, to me, the first sentence is quite acceptable in informal use. In my usage, the tendency to use "best" when strictly speaking "better" is required depends on the sentence structure.
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