It depends on whether we're talking amusement park or theme park because I feel as though I enjoy different elements of both.
Firstly when it comes to amusement parks, I love parks that are relatively small in size, with lots of history and added kookiness which really define the place and makes it feel special. Nothing better than an amusement park that has rides piled on top of one another as well, and there are plenty of great examples of this out there. Another key ingredient of any top tier amusement park is the addition of weird and unusual attractions, be it a one-off traditional dark ride or perhaps a unique flat ride. I don't really think a specific ride type in particular elevates an amusement park into an elite level though. To throw an example out there, I'd say that Liseberg looks like an elite level amusement park (although I can't base this on experience yet). It has all these key elements, plus a number of highly-rated coasters, with lots of interactivity between rides and a seemingly lively atmosphere. To top it off, the park's location is fantastic.
With theme parks I tend to prefer large sprawling parks that seem to go on forever. It does all come down to the whole experience for me, and I enjoy approaching a park and having to travel into each different area progressively in order to discover and be immersed into their unique environments. It's nice when you can tell a theme park has been very precisely planned out for you, as the guest, to experience in a certain way. The detail involved in some world-class theme parks can make the experience worthwhile alone. A few other points to note for me would be: landscaping/foliage, use of colour, use of music and soundtracks, food selection, coaster variety and operations.
PortAventura is an excellent park and ones of my favourite theme parks but objectively speaking I'd say it misses the mark when it comes to being elite level. The only park I'd say I've visited which is up there is Phantasialand, although I adore the layout and use of landscaping and foliage at PA. Really interested to experience Europa in 2 weeks as I feel as though it could tick a few of these boxes.