Just did Heavenly and Mammoth back-to-back, and did Northstar earlier in December. Out of all of them, Mammoth is the obv favorite, but between Heavenly and Northstar, my pick is Northstar. It's not that Heavenly is bad, because on a pow day, it's FANTASTIC and the snow fall is so light and amazing that you won't want to be anywhere else, but at the same time on that pow day, you will be walking EVERYWHERE. I don't like walking. So find a skier to pull you or get your legs in shape.
Really though, Heavenly is sick, and if you go, see if you can get out to Mott Canyon. Awesome riding to be done.
(For full disclosure, I've only been to Heavenly and Northstar in the Tahoe area.)