I collected a ton a character autographs on my past trip and my best finds were at:
EPCOT: all over the World Showcase....even though these characters are announced in the Times Guide, there was little to no wait for any of them if you timed it right. I saw Mulan, Jasmine, Aladdin, Marie, Bellie and the Beast, Mary Poppins, and Alice with less than a 5 minute wait. The only one I had to pass by was Snow White.
Hollywood Studios: Definitely plan to be in the Animation Building around 4:50 on a non-Fantasmic day. Besides the normal characters (the Incredibles, Lotso, and Sorceror Mickey), the random character greetings are awesome. There are two sets of characters, the first appearing at 4:50 and the second appearing at 5:20. It's a great meet and greet because it's not published in the Times Guide so only those "in the know" will be there. Last week I saw, Mary Poppins and Mr Penguin, Capt Hook and Mr Smee, Genie and Jasmine, Pocahontas and Meeko, Minnie, Pluto, and Goofy, The Queen, and Pinocchio and Gepetto.
Animal Kingdom: There were no characters out in the Good Times Greeting Spot, but I had luck in Camp Mickey-Minnie by hitting the character lines about 10 minutes after a Festival of the Lion King show started. I saw Chip & Dale, Thumper and Miss Bunny, and Pocahontas with no lines. Mickey, Minnie, Donald, and Goofy were also there with short lines.
Did not see any characters in the Tear Drop, Millenium Village, or the American Adventure Pavilion.