On the Tahoe area....Speaking as a recent beginner, I spent the 04-05 and 05'06 seasons mostly at Boreal, and after getting creamed by a boarder from the back, I couldn't take the heavy and often out of control boarder (and to some extent, skier) traffic any more and decided to not go back there. I had to spend all my time looking back to check for out of control boarders and skiers. It's truly amazing how many people will barrel down a hill at top speed, yet not have the skills yet to turn or stop adequately. Boreal is really strongly populated by people who probably only ski a couple of times per season and who never progress beyond a low and dangerous level of inexpertise. Boreal's lower slopes are good for learning, but just too darn crowded and the more advanced runs funnel right into the beginning runs so you have people tearing down at top speed among the beginners who are weaving back and forth. Boreal now strongly caters to snowboarders, especially teen boarders, and they are pretty wild and fast. We skied mostly mid week, which during non-holiday times was fairly uncrowded, but weekends and holiday periods were hopeless in my opinion. If you can ski mid-week at the right times, though, Boreal isn't terrible in terms of their terrain and they have a program (free) where you sign up, then after buying two tickets you get the 3rd time free. So the cost is fairly low compared to other resorts.
Mid last year we tested out Squaw and absolutely loved it. It was never horrendously crowded, we rarely spent more than a couple of minutes in lift lines. There are lots of wide open slopes so there is less congestion, in general the skiers dominate over boarders and in most areas people are good enough to avoid slower skiers. I felt a lot safer there and was able to concentrate on my skiing rather than avoidance of a crash. My skiing improved 1000% because of my time at Squaw, and we loved it so much we bought a mid-week season pass for the upcoming year.
Squaw's passes are fairly expensive, but there are other options to save a bit of money....a pass in which after you've bought 4 passes you get the 5th time free. This lowers the overal cost spread over 4 ski trips. You can also get a Snowbomb card for around $30 which takes $10 off each regular ticket all season long ($10 off at Squaw, and varying amounts off at other Tahoe resorts) and if you buy the card at Costco, you additionally get one full-day ticket to your choice of several resorts, including Sugar Bowl, Homewood and several others. I'm not sure if you get this same deal when you buy the card online...perhaps you do.
Squaw has a total beginner area for rank beginners at the bottom of the hill where the lifts all start, but at the top at High Camp are extensive fabulous beginner to low intermediate runs, most of them very wide and open. Lessons are a great value...$49 last year for a TWO-hour lesson and when I did the group lessons there were never more than 3 in the class. (There theoretically could be more, but in my experience classes always looked small no matter what the level.) Can't complain about that, and the instructors were pretty good. They have a Squaw kids program for young kids, but don't know the details on that.
I skied Sugar Bowl once last year (fairly early on when I was still more tentative about my skiing) and found the main beginner run way too flat and easy, and the next more difficult run too hard and too full of speed demons, so once again I was nervous about getting crashed into.
Alpine didn't seem great to me as a low level skier, either. They had one easy run that was way too short, crowded, and it quickly got boring since it was so short. The next more difficult run was too hard for me at the time, and was also long (tiring for a weak skier). Plus, towards the end of the season they closed the lower level runs and only the more advanced runs are open.
I've heard Homewood is good for beginners, also Tahoe Donner. These two resorts seem to have shorter seasons than some of the others.