It depends on the kind of "beach" you want and what you want to do. Surf, dive, swim, sunbathe, tide-pooling, picnic, camp out, hike, see certain widlife (elephant seals), etc. I grew up in SoCal so I happen to think northern California beaches are too cold, the water mainly, but the weather can be nippy even in summer. On the other hand, I love the generally rockier beaches, rugged coast in NorCal for their scenic beauty.
In NorCal (or call it the central coast), I'd say Santa Cruz is awfully nice. Cold water, but kind of a meld of SoCal and NorCal beaches. I like it there in the summer. Of course, I like the boardwalk, too. I love roller coasters.
I adore the headlands of Mendocino. Sunset sitting on the headlands is great. I actually like fall and winter there. You can bundle up in a nice warm sweater, walk the beaches and trails, eat great seafood near a blazing fire.
One State Beach that's supposed to be less croweded and nice is Manresa State Beach. I've never been there, but I'd like to see it. It's south of Aptos (below Santa Cruz).
A lot of Bay Area folk would claim Stinson Beach as nice beach to go to, but I don't know it very well.