This is such a difficult thing to answer, because it can entirely depend on crowd levels. Efteling for example is a park I would call relaxed, but I've had a particular day filled with school trips that was a nightmare and just stress. So I'll try and pick parks which I think would be relaxed/stressful regardless of visitor numbers...
Relaxed
Linnanmaki (Finland) - The park is well designed and has rides that are designed to cope with its crowds. Go variety of rides, good indoor and outdoor selections, good food. It's a nice city park on the outskirts of a city, creating a surprisingly relaxed and chilled out atmosphere.
Hellendoorn (Netherlands) - A spacious park with a good variety sprinkled all across the park. Nice green spaces too.
Stressful
Fantasy Island - The worst parts of a theme park mixed with worst part of a seaside resort. The market and arcade is just noise and eurgh.
Drievliet (Netherlands) - Awkwardly small space and just weird and annoying. Probably slightly basing this on the fact I visited when a million school kids were there which didn't help, but in general, I think this park would just be annoying and stress.
Parc Saint Paul (France) - Visiting the park feels morally dodgy tbh