Magic Mountain is the countries top coaster park. Theres also a good selection of tamer rides such as log flume, rapids etc. The park is huge but some complain it lacks Disney quality theming. If you love coasters, its the park to go to!!
Knotts is very nostalgic. Many shops built in actual old west buildings moved and reconstructed from their ghost town origins. Knotts Chicken Diner has some of the best fried chicken and boysenberry pie you'll ever find. Much of the park focuses on large coasters and other thrill rides but not as many as Magic Mountain. Other rides and shows are a ton of fun as well. Its only 5 miles from
Disneyland making it the most convenient for a side trip from Disney.
Universal is top notch for park and ride theming. Theres only 3 major rides, Jurassic Park, Mummy and Simpsons. All are very well done. The rest of the park is all about movie oriented shows. The backlot tram tour which goes thru old movie sets, king kong, fast and furious, war of the worlds is not to be missed. The WaterWorld show is top rated and always a must see. My favorate is the 4D Terminator show.
So choice would depend on interests. Thrill rides pick Magic Mountain. Balance of thrill rides, nostalgic shops and great chicken pick Knotts. Slower paced day watching shows along with the shops and dining in City Walk pick Universal.