I use Agoda & more recently Trip Advisor. In addition to the ranking number, I always take the time to read through several dozen comments (both good and bad) regarding the hotel. I also take note of who is making the comments (what country they are from and their rating history). I usually do pretty well selecting hotels with this method.
Dont rely on "stars" or some number system. Take the time and read through all the reviews from past patrons.