I think you'd be able to enjoy the park regardless, but the parks itself really does contain elements, or references to all the films. However, if you honestly want to get certain references, or have the ability to watch the films and see things at the park and be all, "Oh, I remember that!" but want to focus on certain films and not all of them, I'd say #3, #7 and #8 (I'd say just #8, but it is a two part film, so you'd be confused to watch one half and not the other). Perhaps, #6 as one of the new shops at Diagon Alley is introduced in the 6th film, as well as you get to see The Three Broomsticks in the film.