The best wine is the one that you like irregardless of what some expert thinks. However, ratings by a reputable group are a great help in deciding whether to buy a wine that is unfamiliar to you. When a merchant provides rating information, they are giving good service to the customer. Wine Spectator and Wine Advocate (Robert Parker) provide dependable ratings.
Wine ratings are based on a 100 point scale. However, most wines fall within the range of 83 to 93. Wine Spectator's list of the top 100 wines contain only a few wines rated 94 to 96 and none rated above 96. The wines above 93 are priced $50 per bottle and up (way up).
I am not sure that I can discern the difference between an 89 wine and a 90 wine, but this is a general guide:
88-89 Very Good
93 and up Extraordinary