Characters are always subject to change, but it seems like the most common ones to see in the parks include: Cruella DeVil (101 Dalmations), the Evil Queen (Snow White), Captain Hook (Peter Pan), Dr. Facilier (Princess & the Frog), and Darth Vader (Star Wars).
The party has a Villains meet and greet by the Main Street Train Station, which usually features three villains (two faces plus one head character). They rotate through the night, but it's usually the same characters for all of the parties, so after reports come in from the first party, we should have a good idea who the characters will be this year. In part years, I've seen Maleficent (Sleeping Beauty), Lady Tremaine and the Step Sisters (Cinderella), Jafar (Aladdin), Hades (Hercules), and/or Ratcliffe (Pocahontas) - not all of them in all of the years, except Maleficent (she's been pretty consistent at MHP).