#1 Brock Lesnar

In the year 2000, Brock Lesnar signed what was at that time the most lucrative developmental contract. Lesnar would run through his competition at Ohio Valley Wrestling and debut on the main roster on the March 18 2002 episode of Raw.
5 months later, Lesnar not only won the 2002 King of the Ring tournament, he also defeated legends like Hulk Hogan and The Rock during this period and became the youngest WWE Champion in history at the age of 25. In addition to this, he also won the 2003 Royal Rumble match.
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I cannot think of another superstar who enjoyed the kind of success Lesnar did in his rookie year with the WWE. It was inevitable that Lesnar would go on to become one of the biggest draws in not only professional wrestling, but also in the world of Mixed Martial Arts.