Every park has lots of great souvenirs. As the PPs said, it really depends on what you are looking for. Each park has unique items that are specific to that park and/or ride. The World of Disney in DTD has the largest selection of items in one places, but they generally are not the park specific merchandise.
On my family's last trip, we focused on finding food-related items. We collected a lot of souvenir popcorn buckets (each park has a unique design), drink mug (shaped like the resort ones), and other items. These are some of my favorites...
The popcorn bucket from AK:
The popcorn bucket from Epcot:
The R2D2 popcorn bucket at DHS (and MK, I think):
The mug at AK:
The Test Track Cone (left, the one on the right is from Carsland at DLR):
The sipper cup with glow cube (not pictured) from Sci Fi:
The barbell and character sipper cups from Goofy's Candy Kitchen:
The Pirate Chest (with cupcake) at Starring Rolls:
If you're Phineas and Ferb fans, there is the Doofenslurper at Epcot (recent reports say their sold out, but they be re-stocked):