I don't think there is a single best site. I generally check if the hotel is a member of marketing organization, then if it's member of a prestigious booking site like Kiwi Collection and finally if it's been included or won prestigious awards. Condé Nast Gold List or Hot List, Travel+Leisure top500 or Tatler. Then I check Tripadvisor (just to check majority of reviews are excellent), then I check other reviews. Google the name property followed my review and see what comes up. What I love most is called www.flyertalk.com, right here, where discerning travelers give their best advices and personal recommendations.