Keeping things reasonable, here's what I'd pick for each of SoCal's parks...
Disneyland Resort: Might be a boring choice, but I do think one of the fancier family coasters Disney has been doing of late would be a fantastic fit to anchor either park's section of the Disneyland Forward expansion.
Knott's Berry Farm: For a major thrill coaster, I'd say a wing would be a pretty solid fit at this park, though I question if there's enough space on the property to install one without major modifications to existing attractions. If the park wanted to go for something smaller and more family friendly in scale, I think a Gravitykraft woodie would be a really nice fit.
Legoland California: Obviously they're not going to get a big thrill coaster, but a family inverted would fit well within the park.
SeaWorld San Diego: I'm not picky about make or model, but they need a proper family coaster with a 42" height requirement as even their more family friendly rides (Arctic Rescue and Manta) are 48". If they have to go more thrilling, it'd probably be best to do another LSM coaster just due to the height restrictions of the property even though they've already got several launched rides.
Six Flags Magic Mountain: Assuming Viper is on the chopping block, I'd love to see a massive tilt coaster replace that as it would be one of the few designs I can think of that would make a similar statement. I also think this would be a good property for one of the family boomerangs if Six Flags continues to purchase those, especially as they're retiring two of their junior coasters this year.
Universal Studios Hollywood: This one is super challenging because we don't yet know what type of sensations will be brought to the park by Hollywood Drift, plus it's very difficult for this park to do major outdoor attractions. The safest best would probably be a copy of Mine Cart Madness (which was planned at one point), but I'm not sure that's what I'd pick as the best coaster for them to install.