Disneyland California and Europa are the only two parks I've been to that I'd describe as perfect. They're quite different, but I think Europa is a better park due to quantity of attractions. Not to say what Disneyland have got isn't perfect, there's just less of it.
These are what I think a park needs to be perfect: excellent operations and throughputs, interesting history, a decent selection of coasters, a large number of attractions, quality dark rides, faultless theming, great shows, events throughout the year and decent food.
But all these things would not automatically make a perfect park. They also need a spark, something ineffable, what is often described as "the magic" in the context of Alton. Those shivers down your back when you hear the main theme of Europa Park Suite. Riding the Matterhorn and knowing you're on the same ride that Walt Disney rode in the 60s. Those little flames of geographical romance that bring a truly perfect park to life as a friend.