In the short history of MMA, there have been a handful of fighters that were once considered unbeatable, and some still are.
Here's a short list:
Mark Coleman -- When he entered the UFC in 1996, he finished his first six opponents, including Severn, Goodridge, and Frye. He semmed like an unstoppable wrestler for a long time until Mo Smith figured out his game and beat him.
Sakuraba -- Before taking his first loss to Silva, Sak went 12-1-1, with his only loss coming from Igor after fighting for over an hour with Royce. He seemed to have developed a style that beat BJJ and destroyed all the best fighters from Brazil.
Frank Shamrock -- Finished his career with an amazing streak of eight consecutive wins, including a TKO over Tito, a submission over Horn, and KO's over Zinoviev and Enson. Frank is often regarded as the best MMA fighter of all time.
Nogeira -- After an impressive run in Rings, Nog came to Pride and won eight consecutive fights, half of them by triangle choke, with only one fight going the distance. The wins included Herring, Schilt, Coleman, Enson and Sapp. He has come back strong after taking a hard loss to Fedor.
Fedor -- Has not lost a fight in the past four years, has 16 consecutive wins over Nog, Herring, Schilt, Coleman, Randleman, Babalu, Arona and many others. Aside from a suspect chin, it's hard to see any weaknesses in his game.
There are others, including Royce and Belfort in their early days, and Vanderlai Silva since his loss to Tito.
So who is the best unbeatable fighter ever?
I would have to say Sak. Even though he has become little more than Vanderlai's punching bag in the past few years, no one has ever matched his legacy.