The best US parks I've been to are

Busch Gardens
Kings Island
Hershey
Kennywood

These have the most personality, amazing rollercoasters, and they aren't too oppressive generally when it comes to crowds or finding shade.

Most of the Six Flags properties are unbearable - they have some great rides but tons of concrete, less character, terrible concessions, and a business model that involves getting as many people into the park as possible with discounts and then trying to milk $ out of them on crappy food and other add ons.

If you are planning on going to Europe, I'd highly recommend Etfling in Netherlands and Tivoli in Copenhagen. Neither have the amount and outrageousness of the thrill rides in the US but they are both just awesome, gorgeous parks.

In my experience, if a park is REALLY packed, it's still a shitshow even if you cough up for the "fast pass." You've just traded one line for another. If you really can't stand lines, you need to avoid weekends at the huge mega parks.

Kennywood is great, but I think the best spot for Wood these days is probably Hershey.

Comet is a classy old classic, lightning racer is a ton of fun, and the Wildcat is batshit crazy. They all look fabulous at night also.

You could slip in the DC area six flags also, the Wild One is a very good ride and there's a couple other things of note. Kind of a dump, usually zero lines, worth stopping by.