the User Reviews seem to be more accurate but RT does manipulate them as well citing "view botting" for any trends that do not match their desired outcome from a film
They would generate that score by looking at other things you have rated, finding ~100 other users who have voted similarly to you, and then showing the scores that those other users thought of each movie you are thinking of watching.
This is much harder to bot, since you will only see the botted scores if you yourself vote like a bot.