I think Whitney Houston had one of the best voices in the music industry (pre-vocal cord damage). She sang smoothly, effortlessly, it was never a struggle to tackle the range she could cover, nor to belt out sound at maximum volume. That kind of voice doesn't occur often. If she hadn't gotten mixed up in drugs, she would likely still be with us today. A female singer from the previous generation who had a fantastic voice and who could also belt out a tune effortlessly was Cass Elliot, but because she came in a weightier package she didn't carry the instant sex appeal. She, too, died prematurely. So, while Whitney had a great voice, she wasn't the only one in history to have a great voice.
With that being said, one of the worst concerts I ever attended was a Whitney Houston concert -- around 1986 or 1987, can't remember the year for sure. I have been to many concerts by performers of various genres in some of the worst venues (rainy day on the track at a state fair), and my reaction has always been that I got as much or more out of the concert as the ticket price I paid, but not with Whitney on that particular night. She was scat-singing a lot of songs (almost all), not to any particular tempo or phrasing, to the point where we thought she was high. I would have asked for my money back, but if the performer shows up and performs for the stipulated time, you can't get your money back. I have never owned any albums from Whitney Houston, but she was undeniably good at what she did -- just not while she was drunk or scoring coke. It didn't surprise me at all to later learn that she was using drugs.