I think any of the Latin dances are "hard" when you work on getting the best technique that you can. I find Bolero very challenging, for instance, when you incorporate rise and fall; and drop and drift.
I found a challenge in Samba as well--but once I "got it" it has become one of the dances that I can do most instinctively. I found it extremely fulfilling after so much training to gain profiency in Samba as it is one of the most enjoyable dances and rhythms around.
Ray Sison
DanceMakers of Texas
Fort Worth, Texas