It all depends on who you would deem to be a rare character. The 7 Dwarves and Jack Sparrow are usually the two most popular M&Gs. People were reporting the line for the dwarves started around 5:30 at one of the parties already. Several of the princesses will have their princes with them, Lotso, Tarzan & crew, Abu and Genie are also on the rarer list right now. Obviously the Villains, but they are for about 15 minutes after their show, and pictures only. The normal characters, Mickey & Minnie, Pooh & Friends, etc are usually in costumes. So while you can meet Mickey and Minnie every day, you can't meet Prince/Princess Mickey/Minnie or Vampire/Witch Mickey/Minnie every day.
KennyThePirate has a good map and rundown on his blog.