In phrases like “pistachio is one of the few flavors that appeals to me,” formal grammar would require the verb to be plural (“appeal”) rather than singular (“appeals”) because “that” acts as the subject for the verb, and “that” in this sentence refers to “flavors.” However, many sophisticated users of English allow for the singular verb in this case, though I would caution against the singular verb if you are taking a test on English grammar.

This is one of those occasions in English usage that lets you follow your ear to determine what works best. If you thought “let” would have worked better in that previous sentence, you would have formal grammar on your side, but using “lets” should not get you into trouble, either.