No one did. He was never in trouble. Maybe stunned or clipped a few times, lost some rounds here and there. But never in trouble.
Lora's right, Monroe seemingly beat him rather easily the first time. If you're asking who had him hurt the most or closest to being stopped, I'd guess Hearns in the first round, but he road right through that.
From what I saw, Hagler had an iron jaw. Only vulnerable to losing a points decision. I don't think anyone could KO him.
Gotta go with Vito in the first fight. Whatever you think of the decision, and I thought it fair at the time and haven't scored it since, the fact is this -- Vito wins one more round and takes a decision and it's a long time before Hagler ever gets another title shot. So while MMH wasn't "in trouble" as in staggered, he was in trouble as far as being derailed that night.
The worm comfortable beat him the first time, and gave him a little bit to think about in the rematch, but the Marvelous One, cracked it, third time round. From what I remember Hagler gave Briscoe a bit too much respect as well.