Southern Californian reporting in.
Winter: 9/10. In general, a great season to go /out/ in SoCal. The weather's in the 60's, rain is infrequent, there's a respite from the sun, and snow is both accessible and generally safe to go /out/ in (save for some mountainous areas like the Southern Sierra). There's also great snow sporting available.
Spring: 10/10. A little warmer, a little sunnier, the chaparral is still green, the air is still fresh: it's usually great. The deserts are beautiful this time of year, Antelope Valley Poppy Reserve and Death Valley are in prime form. Snow is still accessible in some parts of the region.
Summer: 8/10. It gets hot, there's not a lot of water, the landscape has turned brown, and everyone is outside. A lot of places are crowded. That said there's still great opportunities and it's not really that bad. The deserts are to be avoided, but the mountains are pleasant. Just bring water with you.
Fall: 6/10. Personally, my least favorite season to go /out/ in SoCal. It can stay hot through November, there are no fall colors (unless you count dead-chaparral-brown), and Santa Ana winds make things generally unpleasant.
I man come off as a soft grader, but SoCal is in general so agreeable it feels frivolous to grade anything under a 6
