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In the world of professional boxing, staying undefeated is the hardest feat to achieve. While many champions lose their "0" early, a rare few have gone on legendary runs that lasted for decades. These are the fifteen longest unbeaten streaks in boxing history. **Brian Nielsen.** With a 49 unbeaten streak, this Danish heavyweight hero equaled the legendary record of Rocky Marciano by dominating the European scene in the 1990s. He was aiming to break the all-time heavyweight mark in his 50th fight