Mt. Baldy's Thunder Mountain has the best terrain in Southern California, with multiple legitimate black diamond runs and varied terrain: steep groomers, bump runs, some fun little canyons. A few laps on the Thunder Mountain chair are my ideal warmup for trips to bigger mountains.
The lower mountain is even better, but it's rarely skiable because of the low elevation (and climate change).
I'd love for Baldy to have a big year sometime soon so they can make some $$ and invest in upgrades.
I love the Notch lodge. It's perfect for a beer and then a chair ride down.