The most scary KO in UFC ever was Brad Kohler vs Steve Judson, not only was that punch so powerful and knocked him out cold, but Judson was out for a long time, they were having trouble getting him conscious, if there was any fight that was close to being a death in UFC then it was that one. Big John said that was the fight he was most worried about because the doctors couldn't get Judson to come round and he was breathing difficult.
Remember Kohler was only 5'5'' but 250 Ibs of pure muscle.. and he launched all his force into that punch