Killer Croc: A.K.A. Waylon Jones is born with the medical condition, Epidermolytic Hyperkeratosis, that caused him to grow progressively into the likeness of crocodile. He is of low intelligence but has great strength, as well as abilities similar to a crocodile; such as a keener sense of smell and the ability to hold his breath underwater for long periods of time.
Claim to Fame: Killer Croc attempts to cure his condition but when the doctor fails, Waylon Jones consumes her.

The Scarecrow: A.K.A Professor Jonathan Crane is an insane scientist/psychiatrist who specializes in the study of phobias. Dressed symbolically as a scarecrow, he employs special hallucinate weapons designed to instill fear into his victim. 

Claim to Fame: He will occasionally commit robbery, but only to get the funding he needs to carry out his experiments to further the study of fear on humans using the City of Gotham City as his unwilling guinea pigs.

Deadshot A.K.A. Floyd Lawton is often a hired assassin, regularly boasting he "never misses". He is skilled in a variety of weapons, but prefers using a pair of silenced, wrist-mounted guns.  Without a doubt, his most defining trait is the great desire to die in a spectacular fashion.
Claim to Fame: As long as he's been paid for an accepted hit job, he will always carry it out - no exceptions.

The Riddler A.K.A. Edward Nigma is a criminal mastermind who has a strange compulsion to challenge Batman by leaving clues to his crimes in the form of riddles and puzzles. He takes delights in forewarning both Batman and the police of his capers by sending them complex clues. Recently, he has seemingly abandoned his criminal lifestyle and has opted to utilize his skills in order to turn a profit as a freelance investigator.

Claim to Fame: Riddler deduces Batman's secret identity, along with the identify of the late Jason Todd who was once Robin.

Two Face: A.K.A. Harvey Dent has multiple personality disorder as a result of half his face being horrifically scarred by Sal Maroni.  Over the years, he has been reformed at various times, with his face being surgically repaired, only to later fall back into the Two-Face persona.

Claim to Fame: With each of the personalities unable to make a decision on their own, Two Face makes major decisions by flipping a two-headed coin and lets fate decide.

Hush: A.K.A. Dr. Thomas Elliot is a childhood friend of Bruce Wayne who grew up holding a secret anger towards the Wayne Family. Although he went on to Harvard and became a successful surgeon, Elliot continued to hold an irrational grudge towards his childhood friend. While treating Edward Nigma’s terminal cancer, he was accidently informed of Batman’s true identity being his old childhood friend.

Claim to Fame: Hush performs routine plastic surgery on his own face to be identical to Bruce Wayne, in order to ambush and subdue Catwoman.

Bane: An escaped convict from an island prison in South America, Bane has abnormal strength as a result of an experiment with a derivative of the drug known as Venom. He needs to take it every 12 hours or he suffers debilitating side-effects.  After cleverly breaking the inmates out of Arkham Asylum to wear down Batman to the point of exhaustion. Bane waited for his return to the batcave where the two fought resulting in Bane winning the battle and leaving Batman for dead. 
Claim to Fame: He became known as, "The Man who Broke the Bat", when he broke Batman's spinal cord, forcing Bruce Wayne to give up the Batman persona while he recuperated.

Ra’s al Ghul: Translates into "Demon's Head" in Arabic. He is an international terrorist and assassin whose ultimate goal is a world in perfect balance. He believes that the best way to achieve this balance is to eliminate most of humanity. He is aided in this quest by the Lazarus Pits, which are reservoirs of rejuvenating chemicals that restore the dying to life. Due to these pits he has been granted a lifespan of several centuries.

Claim to Fame: He regards Batman as his most worthy opponent, and has frequently sought to make the Dark Knight his successor. He is one of the few criminals who have deduced Batman's secret identity as Bruce Wayne.

The Joker: He needs no introduction; a homicidal psychopath clown that takes comedic delight in violent crimes and playing with Batman like a little boy playing with a toy. He is generally acknowledged to be Batman's arch-nemesis and worst enemy, gleefully citing their relationship as two sides of the same coin. His origin tale commonly depicts him falling into a vat of chemical waste, which bleached his skin and turned his hair green and his lips bright red; giving him the appearance of a clown.

Claim to Fame: The Joker was responsible for the crippling of Barbara Gordon, the brutal murder of the second Robin (Jason Todd), and the murder of Sarah Essen Gordon (Jim Gordon's second wife).

Joe Chill: Not much is known about this individual except that he is, in most versions of Batman, just a petty mugger. However, ultimately this unsuspecting mugger became the number one villain in Batman’s life. Joe inadvertently killed Bruce Wayne (figuratively speaking) that night and created the persona of Batman.  Without the horror Joe Chill committed, Batman would not exist. Why is he in the number one spot? Joe Chill will continue to haunt Batman for the rest of his life and is the only villain that Batman can’t make pay for his crimes.

Claim to Fame: The Murder of Thomas and Martha Wayne.