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UFC legend Anderson Silva says he's NOT the greatest MMA fighter of all time -- telling TMZ Sports that honor goes to Demetrious Johnson.

In his prime, Silva had 17 consecutive wins -- with 10 consecutive title defenses -- leading Dana White to call him the "Tom Brady of the UFC." The dude was a machine.

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But, that was before Johnson went on his MMA run ... ripping off 13 consecutive victories over guys like Ray Borg, Henry Cejudo and Joseph Benavidez.

DJ's the reigning flyweight champ -- and last year, broke Silva's record for most consecutive UFC title defenses.