I've been to all 6 but Seaworld and Disney I haven't been since some of their newest additions.
1. Knott's: Love all their coasters, theming, food, and pretty much everything else this park has to offer.
2. Magic Mountain: Pretty great coasters all around but not much else. Was prettier than the average Six flags but it really has little to no theming especially when compared to the other parks.
3. Disneyland and California's Adventure: I'm not really the best audience for this park but still appreciated it for its thrill rides when I went (Tower of Terror, California Screamin, Matterhorn, and MotorSprings). I still prefer pretty much any Disney World park to either of them though and I also like most of the Disneyworld parks more than MM.
4. Legoland: When I went as a kid I loved this park. When I went again I still loved it. Sure it doesn't have many thrills but I always loved the Miniland and Lego-themed attractions. Definitely not for everyone and not a full day park but it is special to me.
5. Seaworld: I haven't been since Electric Eel but I always liked the landscaping and the two thrill coasters they have.
6. Universal: This place is much more geared towards film production and the studio tour than rides. The rides here aren't bad for say but most are clones or are very out-dated compared to the Florida park.