Independent parks. They are typically the nicest and most enjoyable to go to.
If I had to choose between Cedar Fair or Six Flags, I would choose Six Flags. It used to be really close for me and I had trouble deciding between the two, but after going to more of each, I've found that I just have more fun at (most) Six Flags parks.
Both companies have their advantages. Six Flags parks are very unbalanced as some are great and some are completely forgotten about, as Snoo mentioned. This doesn't seem to be the case with Cedar Fair. There's parks like California's Great America and Dorney Park, but even they are still overall clean and well-kept up parks.
Operations at every park in each company need improving. This is why I am NOT a fan of the locker policy at Six Flags. It's annoying to not be able to bring anything into a park (sorry, $1 per ride is just not an option for me
) but I'm overall okay with it if it makes operations go faster (like Busch)... And it really doesn't! Even with lockers, Six Flags' operations are pretty equal to Cedar Fair. And that says a lot considering some rides I've seen.
Six Flags parks are by far nicer than Cedar Fair parks which are the complete definition of concrete and coasters. At least Six Flags parks have some personality and theming. Kings Dominion and Kings Island are two of my favorite Cedar Fair parks because they have some interesting theming/rides/queues and that's only because they were formerly owned by Paramount. Cedar Fair is already beginning to strip the theming on some rides, like Backlot Stunt Coasters' trains.
Cedar Fair parks are probably better when it comes to flat rides, which a lot of Six Flags parks are severely lacking, but I hate flat rides other than drop towers. So I don't really care either way.
Overall, I feel like I can get a different experience when going to Six Flags parks, but Knott's, Cedar Point and Dorney all basically feel the same (I like Cedar Point for different reasons). I'm also not counting Paramount Parks in this because they were owned by a different company for the majority of their life and therefore do give different experiences... Although KI and KD are very similar.
Busch Gardens and Hershey still win, though.