I would say they're all correct but mean different things. As you know because you have a dictionary and can look them up! However, I would go with "enjoyable" to start with if you're talking to someone socially because you are positive and don't want just anybody to know that you might have found things challenging or hard. "Challenging" is the experience you might confide to a friend or a teacher in order to assess what can be worked on and improved. "Hard" is what you can identify to yourself, close friend or your family with the aim of getting advice and help on how to turn the "hard" into enjoyable.