50. Cassandra Nova

Cassandra Nova was born without a body. So to fix this problem, she decided to copy Charles Xavier's DNA, effectively becoming his twin sister. They grew together all happy-like in the womb until X sensed her evil thoughts. He tried to kill her, but she survived as cell matter and clung to a sewer wall for years, rebuilding herself physically and imitating humans. Out of this brother-sister love, she decided to destroy Professor X's X-Men and his love Lilandra.

Cassandra Nova basically has the powers of Professor X like telekinesis and telepathy. Another more important tool in her arsenal is her ability to block his thoughts from entering her mind. She can mimic the voice and DNA of others, which comes in handy when she pretends to be Donald Trask and gets the Sentinels to kill 16 million mutants on Genosha. She is so powerful that she once took over Professor X's body through the help of Cerebra (an updated version of Cerebro). Then she shot him.

Still in Professor X's body, she forced Xavier Institute student Beak to beat Beast into a coma. She was so powerful, Jean Grey had to split X's conscience into pieces and store them into every mutant's mind.