Best being very subjective, here's some of the better options off California.
I only know Monterey and points south so am not including anything around SFO although the Farrallons are supposed to be good boat diving - possibly with Great Whites.
Monterey has the Kelp forests and all that thrive in them. It's actually a National Marine Sanctuary so a lot of it is protected. If you've never dove in the Kelp, you should it's an amazing experience.
Monterey Bay National Marine Sanctuary Home Page
Moving farther south, there are the Northern Channel Islands, generally acccessible via liveaboard from Santa Barrbara or Venrtura. While all good, Anacapa is a highlight both because there can be Kelp growing there and there's also an active Sea Lion colony and they love to play.
Diving The Northern Channel Islands: The Galapagos of California Santa Cruz also may have Giant Kelp again this year but El Nino has been devastating to the Kelp all over the area in the past decade so IDK how much is back now.
I've heard San Miguel may be the best but we've never been able to get out there due to conditions - plus my friend didn't want to risk his private sailboat.
Then there's the Southern Channel Islands, all of which are accessible by day/weekend boat from San Pedro or Long Beach - you can also access the Marine Park off Santa Catalina via the Catalina Express - a couple times/day ferry that runs between Long Beach, Catalina or Dana Point. They won't be surprised to see dive gear - some divers even bring tanks on a cart on-board.
It's been mentioned that the fast day boats from Long Beach like the Sundivers are almost as good to Catalina than the 2 boats moored there but I've never been on either of those 2 to compare.
Next is the Oil Rig dives - several of the "E" platforms - Eureka, Elly, one more - are diveable. Magician is one boat that does those, there's others. Some are working so often live drops - the depth plus the tangle of the platform brings in big pelagics plus often sharks. It's advanced diving due to currents, conditions and if you get away from the rig they can't always unhook immediately to retrieve you so have proper signaling gear. And be prepared to float in blue water for a time.
in Orange County there's good diving along the beaches, particularly around Laguna Beach. How can you go wrong with a site name Diver's Cove although IMO Heisler Beach or Shaw's Cove nearby are better.
Further south still although you didn't mention it, there's some good diving in San Diego also. There's about 1/2 dozen dive boats - mostly based around Mission Bay who dive locally, the Kelp Beds off Point Loma and there's a small wreck alley with a few deeper wrecks.
Also La Jolla Cove is a world class shore dive/marine reserve and there's about 5-6 other dives within a mile or so. I learned to dive there 30 years ago - the other weekend option there is several of the boats go down to Los Coronados Islands off the Mexican border to dive with the Sea Lion colony there. Divers are their source of amusement so don't be surprised to be shadowed/tugged by a dozen or more - it's normal there - leave the snorkel on the boat...
Usually they board the night before and you wake up on-site and return that night.
It is Mexicp so a visa is required, The ship will provide that at boarding but being from another country, check with them about documentation needed - likely just your passport.
Some useful links:
California Dive Boats : The Official Page
Catalina Express
Scuba Shore Diving Site Listing for: California South, USA West
Divebums - A San Diego Dive Website
I don't have any really good shop recommendations anywhere since we mostly shore dove and got tanks at the Sport Chalet's but they all closed last year and I haven't been to SoCal for a few years, I don't have any problems recommending Beach Cities Scuba in the Laguna area or IB Divers/La Jolla. All the diveboats I've been on do a good job, most have online schdedules/booking on their websites. Don't assume tanks/weights are on-board since a lot of SoCal divers bring their own but all can provide them. Almost w/o exception the DM stays on the boat normally unless someone needs help - you can hire a private guide thru most of them if desired.
hth,
One last thing I'd mention is in the greater LA area, it's not the distance but the
traffic that determines how long it takes to get somewhere so download one of the local traffic apps - there's almost no way to predict the traffic jams except generally anything to/from downtown is bad and to/from the Long Beach area due to the container ship traffic - it's one of the busiest ports in the US. I've never had problems in Orange County but I guess I'm very lucky,