It's got to be the Rumble in the Jungle, in the seconds leading up to the KO in round 8, when Foreman was clearly shagging and you could see the intent in Ali's pinches. He was a man of destiny that night in Africa.
The last few moments of Douglas/Tyson were also similarly electric. Both men had been down by then but it was Douglas who just kept coming. I've never seen a fighter with such limited skills, fighting a top level, elite world champion with such tenacity. Douglas was willing to die in the ring that night.