Certainly Park 4. I feel it had the most relevant designs to the parks. There were no stretches like aquaramouse in park 2. It also had Epcot Center, which is one of my very favorite vinyls. The designs were bright and colorful, and I think they did a great job. If they keep the relevance that they kind of strayed away from in some other series, and add straightforward "i know why that'd be in the parks series" vinyls, they'll sell like crazy. They also balanced the designs enough so that hardcore fans as well as the guests could enjoy them.
And look at the 9"! You can't beat Tiki Room, Sci-Fi, ATIS, and World Showcase Epcot!! They're just so colorful, and, as I said, REALLY represent their icons well.