Interesting question!

You'll get a variety of answers, I bet!
If the trip is somewhat soon, I say MGM. MGM can a big variety of characters by the Big Hat. These characters rotate in and out roughly every 30 minutes or so. If you pass by every 30 minutes, there could be different characters out. Though sometimes you can hit a boring day and just see common characters like Goofy, Pluto, Minnie, Donald, Tigger, Pooh, Eeyore, Chip, Dale, and Stitch. If you are there on a good day, it's possible you could see more interesting characters like Clarice, Lilo, Chicken Little, Jafar, Cruella, White Rabbit, Queen of Hearts, Mulan, Pocahontas, Snow White, and more!
If you wander around MGM and get there early (well before the parade), you can find the Power Rangers, some Little Einsteins, Mr and Mrs Incredible, and Jojo and maybe Goliath. Mike and Sully may be out, too, plus visiting Al's Toy Barn you can find Buzz, Woody, maybe Green Army Man and Jessie. MGM can be hit or miss though, depending on who's out on any given day! But it has good potential for having many different characters to meet.
And like Nickeymouse said, Epcot's World Showcase can be a very fun character place, as there can be characters in so many of the countries! The nice thing about Epcot is that the characters stay out later than in other parks. Often in the other parks, the characters stop meeting before the parade and never return, but at Epcot, they tend to meet throughout the afternoon.