Generally, you should read the word in 2nd syllable, especially when the word appears in the sentence. Just in the time when the word appears alone and in an informal situation, you can read the word in 1st syllable. For example: 你喜欢（xǐhuān）我吗？--------in the middle of a sentence, use the 2nd syllable 喜欢（xǐhuan）-------sigle word used here, but that's all for the answer, and it's a joke,so the 1st syllable is ok ; if your answer is "是的，我喜欢你",you should say in the 2nd syllable; or you just answer"喜欢",but you mean it seriously, you should use the 1st syllable.