im born and raised in the area (23 years) and can say that kirkwood and squaw easily have the most challenging in-bounds skiing in the area. i have never held a pass to squaw, so i cannot speak as comfortably about it as i can with kirkwood, but i do know that i have never been more challenged than i have at squaw and kirkwood. eagles nest at squaw is insanity.
skied mt. rose this year and had some of the best powder days in years skiing the chutes. there was one day, a saturday, where the resort had been closed for five days due to the highway being snowed in, and finally they opened up the chutes. seven feet of powder felt pretty damn good.
kirkwood is amazing and my favorite resort in the tahoe basin for so many reasons, even after vail acquired it. i wont go into detail but if you can make the drive, its always worth it.
heavenly is pretty shitty unless you know where to go. killebrew and motts canyons are the most challenging zones and are comparable to some areas of kirkwood and squaw. the mountain is insanely huge and like i said earlier, if you know where to go, you'll have a good time and can avoid the masses. three and more years ago the park was amazing, the last three seasons it has gone sharply downhill as vail caters more to the vacation clientel and less to the locals.
northstar arguably one of the best parks in tahoe. last season they had at their max 82 features, non stop top to bottom on a high speed 4 banger, giving you a run lasting like 3-4 minutes without stopping of non-stop features. its amazing. other than that, its basically shit. there is a sweet zone called "the stash" which can be great after a storm.
boreal is god awful if you arent trying to ski park. if you are, youll have a really good time and can ski from like 9a-9p. plus its cheap, and the vibe is unlike any other in tahoe, sans sierra or kirkwood.