It all depends on what type of skier/boarder you are. Sierra at Tahoe is more for beginners/intermediates, they have a great run for beginners, one of the longest Green runs called Sugar and Spice. Their backside is decent with a bunch of blue runs, but pretty short in general.
Northstar is for for everyone I would say. They also have a pretty decent green run but you have to take the gondola back up if you want the really really long green run. They have okay blues, and their backside is all black, few runs have moguls. I say the backside is more for the skiers, less for the boarders.
Kirkwood is really nice, probably my favorite place to go. Though you need to keep your speed alot of the time or you will get stuck near flat areas. Kirkwoods backside bowl is the best, you can get so much speed on that run, I forgot the name of it.
Squaw, is very very nice. Nice and open runs, you dont really even have trails since the place is so open and free of trees (well most places anyhow) probably my 2nd fave place next to kirkwood. But it's damn expensive. One cool thing they have is night skiing. The main run has lights so its really sweet, only thing is, its freezing at night so make sure you have plenty of clothes!
Heavenly I heard is more for skiers since there are steep slopes with lotsa moguls. I've never been there but thats what I hear.
I'm gonna head to the slopes this weekend, should be some nice new fresh powder.