With a cast full of demigods, trained martial artists, spies, mercenaries, movie stars, ninjas, emperors and empresses, and cyborgs, I wish they did more to give Baraka or Reptile more personality than just wanting to gnaw at their opponent's bones. It seems like they've been purposefully treated like fodder since MK9, especially Baraka.
The real answer, however, are the newbies from Mortal Kombat 4. Reiko, Jarek, Fujin, Kai, Shinnok and Quan Chi were incredibly lame. The last two have been making a comeback, but compared to Shang Tsung and Shao Khan, they pale vastly in comparison.