You've got 3 1/2 choices for SoCal:
It's the closest resort, but it tends to be REALLY crowded, and attracts a crowd that can be frustrating. If the East resort is open, there's some great cruising, otherwise the west resort is smallish and mostly park oriented.
They have the most fun terrain (especially if the canyons are open, but that isn't very likely). Silver Mountain and Geronimo are both lots of fun. However, the resort has lots of terrain features, so if you're not into park, or you really like wide open runs you may find the layout frustrating. Also, Bear attracts lots of snowboarders, some of whom come with attitudes (although it isn't as bad as Mountain High).
Summit has more skiers than any other mountain in the area, but the runs feel kind of short and the terrain just isn't that exciting (kind of flat, typically overgroomed). Summit tries to be the most resort-like, but I've found their customer service to be weak.
This is the half resort. If they have snow, this is the place. Steeps, trees, bumps, etc. However, it takes a lot of snow to really get this place up and running. If the snow report says that you can ski to the bottom then go for it. Otherwise, don't bother. Also, bring rock skis no matter what the ski report says.
I should probably mention that I used to live in SoCal and worked at Bear Mountain, so I'm a bit biased. Also, if you don't mind the drive, you might manage to get in a day and a half at Mammoth, but it's 5 -7 ours from LAX to Mammoth (depending on traffic). Mammoth is a major league resort, and is worth the drive, even for just one day of skiing.