I usually tend to see the most at MK. There always seems to be a bunch in Town Square, the crowd in Toontown, and lots others spead out through the lands. Lines can get long when the parks are busy, but all in all MK's my favorite for sheer number of characters and variety.
MGM is next IMO. They have a great variety and a lot of characters you won't find anywhere else (Chicken Little. Monsters Inc, Power Ranger, Jojo's Circus and Kim Possible characters among others).
AK has a good number of characters. The main entrance usually has a bunch and Camp Minnie-Mickey and Discovery Island also have a good amount.
EPCOT usually seems to have the fewest, but also has the shortest lines. This is IMO the best place to find Princesses like Belle, Jasmine, Aurora, Mulan, Snow White, and Mary Poppins with the shortest lines. They're often hidden in the back of the World Showcase pavillions so lines are no where near what you would find for the same characters at the other parks.