Squaw is by far the best mountain at Tahoe. Imagine 4000 acres of the Vail backbowls and that's pretty much Squaw. For some reason it seems to be largely a skier's mountain, but I think that's because of the lack of a good terrain park. Go to Northstar for that. Squaw is a mountain where you just pick a lift, get off, and go wherever your heart desires and your skill level allows! There are definitely amazing and challenging runs on lift-accessed terrain - KT-22, Silverado, Headwall, Granite Chief, not to mention the hike-to Palisades - a section I know I'll never ski - I know my limits!
The down side of Squaw is the tired but true accusation of poor and rude service. The on mountain facilities are really dated and it's impossible to get decent food the mountain. It's a mountain with so much promise that just doesn't deliver. That is, except for the skiing. Lift lines are relatively short and the skiing is amazing. I do wish they take a lesson or two from Northstar, but I'll keep going back. It's the best skiing in Tahoe, hands down.
Don't get me wrong, I love skiing period and am happy on any mountain. I love the local favorites Alpine and Mt. Rose - great atmosphere and awesome skiing. Heavenly is an unwieldy, disorganized, poorly laid out mountain with huge lift lines packed with way too many gamblers testing their poor mountain skills, but like Kirkwood and Sierra can be fun if you want a change of scenery. Sugarbowl and Northstar are both great places to warm up and get your ski legs back (Sugarbowl has also got some pretty challenging terrain) - and have the best terrain parks by far at Tahoe. Ski season doesn't start, though, until I'm at the top of Granite Chief and choosing my line down back at Squaw.