Ideally, I'd love for Disneyland to be the first park I'd go to. It's been a good while since I've been, and I'm dying to see Star Wars Land. However, it'll most likely be Six Flags Great Adventure. With Jersey Devil (hopefully) opening up, it's an obvious choice for me.
I will be honest though, I am probably going to wait a while until I return to a park. Even if they open quickly after quarantine is done, I'll probably still avoid them since theme parks are basically a perfect hotspot for this virus. Even with the best intentions: cleaning methods, masks, hand sanitizer, all of it, the sheer design of a park is based on the idea of people being grouped closely: in lines, on rides, in midways. They could implement a social distancing initiative, but the virus can live on surfaces for hours or even days. Shared surfaces are an inevitability in a park, and I can't imagine them being able to clean every single surface after every use.