I had to go with the musical instruments. I love hearing their playing improve from ear-shattering-screeching-car-brake to Itzhak Perlman bringing people to tears with the theme to Schindler's List. The difference between levels 1 and 10 is so stark, and there's an audible and visible improvement with every +1 in skill. Other skills may be more rewarding in the Simoleons department or for other practical reasons, but to me, instruments always win out for being the most rewarding.
I like the Singing skill for the same reason.